I think today's Dragon Age RPG, where we had a guest playing one of the NPCs, was probably one of our best sessions to date. I think we got lucky -- Liz was a newbie to RPing and we kind of lucked into a situation where not only did she get to take an NPC who was relatively straightforward to play and had an interesting background, but also had an opportunity to be center stage for a portion of the session. That made for an excellent experience!