Scared Straight... Off Balance
One of my students, Ben Kerr found this fine montage of people being scared by practical jokes. The interesting thing with most of these clips is that the people react the same way. Sure the patterns of movement differ, but most often The "victim" of the practical joke loses there balance.
Next time you have to animate a take think back to the extemes of surprise shown here. I'm not suggesting you animate the character always falling down, but I think we can take from it at least one thing. When frightened, our brain processes things in a certain order.
Brain's Thought # 1- Yikes! Let's get away from that which frightens asap.
Brain's Thought # 2- Falling! Must try to catch our balance.
Brain's Thought # 1- Ooof! Too late. Sorry about that pal, but that really freaked me out. I mean damn, I really didn't see that coming.
The reaction time is so quick that the need to flee and the need to not fall down compete directly with one another. It's as if when scared, our body naturally falls down for comedic effect. Weird. Also I think that the cats are funny.