Oh, this takes me back to the 90s.
Used to play DSA (very popuar German P&P game, The Dark Eye in English). One of my longest lasting player characters was a priest of Rondra, goddess of war (basically a Paladin kind of character). His name was Thoralion and he had a very rigid sense of honor. Thus he clashed frequently with one of my friends characters, a rogue dwarf, shooting people in the back with his crossbow.
However, my favorite time was DMing the Borbarad campaign. It is by far the best campaign I have ever seen in P&P but it does require a fairly large group if you want to play it right. So as a DM, I used one of my player characters, a mage, to accompany the group as an NPC. It worked great because we used to shift DMs around, so I had already played the mage within this group before the campaign started. He could nudge them in the right direction when necessary and was very useful at times but I decided to give the whole thing a bit of a twist. This campaign exposes the group to black magic a lot and in fact, the main Antagonist is the most infamous black mage of all time, returning from an eon long imprisonment to conquer the world. Over the course of the campaign (which lasted over a couple of years game time but also real time to play) my mage would slowly come to the conclusion that he had to fight fire with fire. Starting out as a grey (or neutral) mage, when the fight became desperate, he became a black mage himself and I altered the last bit of the campaign a little, so that he and his dark powers played a small but pivotal role to defeat the big bad.
However, some time later, after the heroes enjoyed their victory, they had to set out again since my mage, succumbing to the dark powers that he wielded became a threat to the world himself. The heroes had to kill their old friend, who had become evil despite the fact that he always had good intentions.
It was great to have a group who would actually go along and play out the tragedy of the situation.
Oh, and here is the funniest moment we ever had: We used to hike out to an old cottage in the countryside for weekend P&P sessions. The cottage belonged the grandfather of one of my friends. No electricity, you had to get water from a nearby spring in the woods and the whole thing was furnished like an old hunting cottage (trophies and such included).
So when we were there for the first time, I was DMing that particular session and just before midnight, the guys were coming up to the final fight (this was also some sort of black mage, I think). So here we were, sitting in candle light and I was describing the situation: "You enter the dark cavern, only lit by the shimmering flicker of your torches, shadows dancing on the walls. Suddenly, the narrow passageway opens up into a chamber. It is damp and warm. A faint mist is hangs in the air, thickening just above the floor. In the middle of the room stands a figure, clad all in black robes, his hooded head lowered in prayer or meditation. As you enter, the mysterious stranger raises his head bu you cannot see a face beneath the hood, just shadows and two eyes, glowing in an incandescent red. The men raises his hand high and says: "
THIS! (click the link, this cannot be written down) Turns out, it was just midnight and this guy's grandfather had put up a cuckoo clock in the cottage, which went off in exactly that moment. Needless to say the suspense was gone but we all had a pretty good laugh at that one.
Ah, good times.