Megan has her semi-formalized process of writing down motivations for each NPC, and I take a similar (but even less formal) approach. I like to think of it as "pushing buttons" on the player characters. If you understand the characters and the players well, it's like pushing a drama button.
Let me just quickly switch metaphors, to complicate things unnecessarily. Character backstory, problems and motivations are a whole big tapestry -- they tell you everything there is to know about the character up to this point. Drama is about destabilizing that "big picture" of the character and forcing them to take action, make a choice. Tugging on a string to see what unravels.
Often, as a GM you'll have one or two BIG character "buttons" (or strings, or whatever metaphor you prefer) that you want to push in a given session. And you should push them HARD when you do. If it's a character's spotlight episode, they should have all kinds of unpleasant decisions to make and come out of it smarting.
What I like to do, to give my episodes some texture (or adjust pacing on the fly, if I find that things are dragging and we need something dramatic to punch it up), is to have a few ideas of smaller "buttons" that I can tap on as sidelights to the main event. There's a character that someone doesn't get along with? They show up. Two PCs are having a fight about what's the best way to proceed on their Master Plan? Throw them into a scene together. We haven't seen a character's conflict on screen for a while? Toss them a little reminder, a little button press to wake them up and remember the struggle to come.
This works especially well if an episode concentrates on one character's conflicts -- you can still have a little scene for one of the other characters that makes it worthwhile and memorable for those players too.
Modern games like FATE make this very easy, because character generation formalizes PC issues ("buttons") in a way that makes them very easy for the GM to grasp and refer to quickly. An Aspect with negative / Problem overtones is a ready-made, player-generated button that says "Push me for Drama, please." You can also get some mileage out of placing Aspects that aren't usually negative in crisis -- if someone's an expert swordsman, they should occasionally get to face down someone who doesn't give them the proper respect, to show off their skills. This isn't as interesting as a scene where a PC has to make some hard choices or make a sacrifice to get what they want, but it reinforces the character "on screen" in a way that's satisfying and reminds us of the character's basic focus -- the way Indiana Jones occasionally gets to show off his vast knowledge of history and ancient artifacts in between punching Nazis in the face.
One last thing: It's always worth remembering that sometimes you don't need to push any buttons. Your players will do it all by themselves, if they're sufficiently motivated and focused.
The GM just needs to be ready to push a button when necessary to keep the drama level high.