So I've been thinking about something, and I'm wondering: as far as like "loyalty quests" go in this game, I'm sure we'll have them for our companions. But what about romanceable NPC's(which, heaven forbid, Cullen might be)? I always felt that loyalty quests were kind of a large part in initiating romance with the companions, but if you're romancing someone that's not a companion will there be some kind of fetch quest for them or something? Thoughts? 
Hmmm...
My first thought is that there is a quest you have to do for them, but you do it without them. I'm imagining Varric being chirpy about how badly the Inquisitor wants in NPC's pants the entire time. This could work, but it does have a bit of an empty feel to it. Of course, the benefit of Cullen is that he is a warrior in his own right, so perhaps he could tag along. Temporary companion, or the fifth-wheel NPC spot.
Or, like in ME3, just talking to Cullen often and picking the right dialogue choices would be enough to trigger it. Perhaps there are optional dialogues that can be had, but only if you poke around the base for where Cullen is lurking.
Perhaps it has more to do with his advisory skills. Maybe getting the military agents up will open more dialogues with Cullen, including a romance?