It's easier to play a dim character, for me, because I have a natural bent for comedy. It's not intrinsic for me to be crafty, so I would have to go outside for a source of origin. I think of myself as pretty dim.
When I worked with Robin Williams, now there is improv! He is just as funny as you think he is. We did at least five or six takes of every scene, improvising every scene differently. He was a riot.