One writing exercise that I've heard about quite a lot when it comes to getting your characters' social skills, relationships and cognitive processes down is the "take-your-characters-to-the-zoo" one. Or just The Zoo Exercise, if you will.
Basically its another way to write Bearilou's exercise: you send your characters to the zoo and see how they behave and interact. Which animals would scare them, and which are they interested in seeing? What do they think about animals in cages, about the different types of people they'll encounter (children, adults, staff and so on from different social circles) and so on. How judgmental are your characters? If they saw a crying child, would they buy him/her and ice cream, ignore it in awkwardness or maybe berate the parents?
Since I usually write character driven plots, I tend to focus on who they really are if you strip away the quest/main goal, the history and the possible love interest.
If you tend to write plots that resolve around your characters reacting and/or taking counter measures (say, trying to thwart some disaster), maybe you could do a writing exercise about chain reactions and consequences.
Start with something simple: maybe your character stopped on the way out to tie his shoelaces. That had the consequence of him meeting the pesky neighbor in the stairwell. That made him angry: what could that influence? Maybe he used a bit too much force to unlock the bike and thus broke something that- well, I think you get it.
I find that forcing myself to look at what the small details and actions influence later down the line can help me find my muse again. Read through your last paragraph or two where you got stuck in you WIP.
That tiny little action or description that you put in there to ad some depth and color?
It might be your clue to something a lot bigger. The angry neighbors may have kept your character up all night, maybe your horse didn't get its hooves picked and now its lame, maybe the car is a brand new ride that someone just HAD to scratch with a key.
