A.I. Tactic Behavior:
This is where I will break down how the Artificial Intelligence uses each ability. For the most part we see a level of competence out of the a.i., but nothing is perfect. With the inclusion of Friendly Fire, the list of abilities you can trust to this system shrinks significantly. This will be a comprehensive list. And just as a precursor, use manual activation of Focus Abilities.
*** = All Good
** = Requires you to work with the spell. i.e. Fire/Ice Mine - You control the tank while avoiding the area of effect.
* = Not a chance. It hurts you more than the enemy.
This will be describing a situation where FF is enabled.

Mage:

Barrier: *** - Great Defense; Average Cost; Great Cooldown: Get the upgrade and forget about the use. A.i. does such a great job with this one that you only need one mage with it prior to level 10. (upgrade & Prioritize)
Dispel: *** - Great Utility; Low Cost; Low Combo Damage; Great buff: Required against Despair Demons and awesome at removing effects from your allies, like burning, frost/freezing, and the occasional lightning ring. This will also stop enemies from spawning during rifts if cast on their spawn points. Possibly the best ability in the game. (upgrade - Disable)
Mind Blast: *** - Great Utility; Low Cost; Low Combo Damage; Knockback: the a.i. doesn't force the use on this one, it will use it when it's needed and not before. This will affect your people, but only slightly, no damage, only a slight disorient. Not as effective as it was in Dragon Age 2. I wouldn't say it's worth the upgrade. (the upgrade does not work)
Revival: *** - Awesome Utility; High Cost; Long Cooldown: Great ability that saves you from having to spend time picking people up off the ground. Great buff on the upgrade. I should also mention that it works very well with Simulacrum from the Necromancy tree when timed correctly. (upgrade & Prioritize)

Energy Barrage: *** - Good Damage; Great Debuff; Medium Cost; The only downside to this ability is the cast time. Does break Winter Stillness buff. (upgrade) (stack this with Conductive Current for an excellent damage increase for your staff type.)
Chain Lightning: * - Will use your tank as a jump to get to an enemy. If there is only one enemy it will go back and forth between the 2 until the spell ends. Wrecks your party if an enemy is close to the group.
Lightning Bolt: * - Low Damage; Low CC; High Cost; I dont really see a use for this particular spell as the negatives outweigh the positives on it and there are many other places with better value for an ability point.
Static Cage: * - Low-High Damage; Great CC; High Cost; Traps your melee along with the enemy. This also triggers the damage on anything taking damage in the area of effect. Same situation as Chain Lightning except you can control this one. (Manual activation only.)

Flashfire: *** - Average Damage; Great CC; High Cost; 16 second fear on a single target. (upgrade)
Fire Mine: ** - Excellent Damage; Good CC; Low Cost; Have control of your tank at all times when allowing the a.i. to use this ability. It's an amazing effect that you will love watching up close. (upgrade)
Immolate: * - Great Damage to cost ratio, but extremely dangerous to allow with Friendly Fire. Not recommended.
Fire Wall: ** - Good Damage; Great CC; Low Cost; This one is all about the CC fear. The upgrade is nominal at best. More testing needed for this one, depending on how the fight turns this can be very helpful or extremely dangerous based on your tank's position to your a.i. mage.

Winter's Grasp: *** - Average Damage; Great CC; High Cost; Must have ability on every mage. Disable via tactics vs Cold Resist enemies. (dont upgrade)
Ice Mine: ** - If you're controlling the tank this ability is incredibly powerful combo setup, cc and debuff. (upgrade)
Wall of Ice: * - If you're looking for complete chaos from your ranged characters, this is the spell for you. when it's cast and cuts off line of sight from your enemies, the a.i. forces your ranged to move up to the wall's corner in order to auto attack what's behind it. You can, of course, have them attack targets on the near side if there were any, but with the random nature of their casting patterns there is no way for you to predict where this will land during combat. Manual cast only.
Blizzard: * - No. Just No. far too dangerous to use.

Horror: *** - Great Utility, Excellent Debuff, Medium Cost; The aoe and debilitating effects are enough to warrant this alone, the armor reduction is just a cherry on top. Great skill. (upgrade)
Simulacrum: *** - All the passives in this tree will be picked up, I just wanted to point out the ability to use Simulacrum with Revival to pick yourself up. If you time this correctly you can self revive. How cool is that!
Spirit Mark: *** - Good damage, Great Utility, Medium Cost: This is an automatic pick up. If you have issues with the spirit blocking rift closures or cutscene triggers because you can't turn it off, switch characters then switch back and try again. Turn the spirit off by reactivating the ability. (ugrade)
Walking Bomb: ** - Good Damage, Fun Ability, High Cost: Great fun with the upgrade. Get the target and any other enemies near a ledge for a great time. This will damage and knock down your allies, but any barrier or guard will protect them. The virulent portion will not affect allies. Still Suggested. (Upgrade)

Spirit Blade: *** - Great Damage; even better damage vs guard; low cost; This will pull and keep the mage in melee combat but since this ability does so much damage the passives in this tree build tons of barrier in order to keep the mage safe.
Fade Cloak: *** - Excellent behavior from the AI on this. Good timing and consistent use at the right times. Great skill. Do not upgrade it however as this is very dangerous to your team.
Disruption Field: *** - Excellent skill, Perfect use by a.i. on this. Great lead in to WOMBO Combos and setting up areas to kite large groups of melee through. (upgrade)

Veilstrike: *** - Great debuff and interrupt when you have all the passives in this tree. No negative effects on allies. (upgrade)
Stonefist: *** - Great damage and interrupt utility. Stacks well with Pull of the Abyss. (upgrade)
Pull of the Abyss: ** - Great area control except when against any type of "Brute" or those invulnerable to movement impairing effects. Quite dangerous when left to tactics against "Brutes".
Firestorm: * - Nope. Manual activation only. This will wreck your team.
Warrior:
From all accounts the 2h warrior is fairly squishy on NM so we will be sticking to the Sword and Shield option for his description. It's what I use and suggest for this difficulty environment.

Shield Wall: *** - vs 1-2 normal enemies this works very well, from bears to Dragons. This only starts to run into trouble against brute types; Brutes, Giants and anything that has large knockdowns. It's always best to kite those types as avoidance tanking is the best way to handle such encounters. Vs larger groups of at least 4, the a.i. simply becomes out flanked and the value of this becomes marginalized because of incoming damage.
Shield Bash: ** - the a.i. does a great job of recognizing combo opportunities like frozen or stunned but does not use this freely against armored targets.
Payback Strike: ** - slow combat animation, very effective at knocking enemies down, short cooldown. You get great use out of this ability but the animation and timing used by the a.i. can leave it vulnerable against enemies that can't be knocked down, which are the ones you should be kiting with the tank anyway.
Lunge and Slash: ? - need to test.

Grappling Chain: * - open use of this is discouraged as it's animation time can be lengthy and if used against immune targets simply wastes traverse time to the target. Best for situational manual use so you can jump to the tank and pull someone off your squishies.
Combat Roll: *** - great use out of this one, it may have the effect of dragging mobs into your ranged characters given the random nature of combat and the range on this ability.
Horn of Valor: *** - Awesome Damage buff when upgraded; good use; medium stamina cost; the a.i. doesn't have problems with spammable abilities like this. (upgrade)
War Horn: *** - Good use; the ability itself is very strong against almost everything. It's a rare creature that is immune to fear, typically just dragons.

War Cry: *** - Good use out of this ability. Great guard building ability. Must have. (Prioritize)
Challenge: *** - Great range; great stamina gain; long animation. a.i. uses this very well and spams it often. (upgrade - Prioritize)
Charging Bull: * - horrible use out of this, do not allow the a.i. to ever use this.
Unbowed: *** - Great ability; free to use; awesome guard builder. (upgrade - Prioritize)
Bodyguard: *** - Great ability; low cost: great damage mitigation when upgraded. (upgrade)

Spell Purge: *** - Excellent Detonator, low cost, great utility - the a.i. will not recognize when to use this spell for utility. It will only recognize combo detonators, but the damage caused by using this to detonate is extreme. Great ability! (upgrade - Prioritize)
Blessed Blades: *** - Great ability; low cost; constant uptime. a.i will spam this as often as it can. (upgrade - Prioritize)
Wrath of Heaven: *** - Awesome Damage, Utility, and combo set up. This will stun allies, but will not damage them. In combination with Spell Purge, this combination will near 1 shot most groups of standard enemies. Demons don't stand a chance. (upgrade - Prioritize) Do it Now!
Rally: * - manual activation of focus abilities only.

Line in the Sand: ** - Great Utility, low cost. - A situational ability that can reduce flank options for the enemy but really only good if you are controlling the tank yourself.
Walking Fortress: *** - The reason for this skill tree. Top 3 abilities in the game. The a.i. uses this when ever possible, which can mean when there isn't any need for it. Like during a dragon's Wing Buffet when the tank is under the dragon and safe from damage for the duration of the ability. But at other times it will trigger this just before a fireball hits. (upgrade - do not prioritize)
To the Death: * - do not let the a.i. use this.
Counterstrike: * - manual activation only. I don't even bother with this ability. The point is best used elsewhere.

Ring of Pain: *** - works well enough, especially when stacked with Static Cage. a.i. does not disable the ability.
Rogue:

Twin Fangs: *** - Great damage, average cost, essential. (upgrade)
DeathBlow: *** - abusive damage, average cost, ridiculous damage. (upgrade)
Flank Attack: *** - Awesome Utility as a 2nd stealth trigger and avoidance ability, low cost, low damage. Stacks very well with Shadow Strike and Stealth. (upgrade)
Spinning Blades: *** - High Cost, High damage possibility; a.i. uses this ability at will so it could leave the rogue stamina hungry. Fire Flask remedies this of course but only up a third of the time. (upgrade)

Leaping Shot: ** - Low cost, huge variance on damage and effective use from a.i.; also tends to send the rogue out of the fight by jumping off cliffs, ledges, what have you for the remainder of the combat encounter. The upgrade is nice, but not if you're going to bother using this troublesome skill. (upgrade if you plan to use it)
Long Shot: *** - Great damage, average cost, Great Detonator; everything as advertised. (upgrade)
Full Draw: *** - Great damage, long animation, upgrade is negligible considering the target will in all likelihood be under attack by your entire team.

Poisoned Weapons: *** - Absolutely Necessary! the upgrade is 25% damage period not just to the poison. (Upgrade & Prioritize)
Caltrops: * - Utterly useless. Disable
Toxic Cloud: * - Dangerous to your tank with FF, also the passive on the way to this ability causes the same issue. Do not pick this up with FF.
Throwing Blades: ** - ok damage, average cost, sundered effect can be gained elsewhere with better investment along the way in the Subterfuge tree. Not worth the ability points.
Hook and Tackle: ** - an ability that puts your rogue next to the enemy from just about any range. good if you're a dagger rogue, bad if you're ranged. Great utility, but very bad to let the a.i control this volatile ability. (upgrade)

Stealth: *** - Get it, upgrade it later. (upgrade & prioritize)
Evade: ** - a forward roll that can get the a.i. into more trouble than it should be in. Rely on stealth to clear threat and escape, the cooldown reduction from Shadow Strike will keep you in good favor as dagger rogue. (disable)
Knockout Powder: ? - testing required. Usually not enough points to get full Subterfuge tree. Points spent on left side in order to pick up Ambush which stacks well with a Blinking Rogue.
Shadow Strike: *** - the reason we are here. Great damage, Great utility on the cooldown reduction, Great Detonator. This is a melee attack, do not put on ranged character. (upgrade) do not prioritize

Spike Trap: *** - required. works fine. dont bother with the upgrade it's not worth the ability point.
Fallback Plan: *** - a rogue that can heal himself. get it. do not upgrade it. the point is to heal, not to isolate Varric with any enemy. (DO NOT UPGRADE)
Elemental Mines: *** - Great skill, Great damage, Great defensive measure. (upgrade)
Hail of Arrows: *** - manual activation required to avoid wasted use.

Hidden Blades: *** - Great damage, no risk, high cost, long cooldown. Honestly this works better from a Masterwork item than an activated ability. (upgrade)
Knockout Bomb: **? - have to pick it up on the way to get Gaps in the Armor which is essential for this build. Unsure of it's effects on teammates, will test soon.
Mark of Death: *** - Doubles all damage dealt to a target in the 8 second window. Get this now. (upgrade)
Cloak of Shadows: * - manual activation only. there are much better focus abilities, not worth the investment.

Flask of Frost: *** - Awesome damage reduction, low cost, great duration with passives. the upgrade will cause problems if you are attempting to maintain control with your tank. (do not upgrade) (Prioritize)
Flask of Fire: *** - Awesome Utility, low cost, great duration with passives, Abusable. Get it first, upgrade it right away. Your first 2 points in Tempest go to this skill. (upgrade & Prioritize)
Flask of Lightning: *** - You are the Flash. Get it, upgrade it. (upgrade & Prioritize)
Thousand Cuts: ** - manual activation only. Best Focus ability in the game. Damage is unreal.