When people draw animations of stick figures what do they use brush or line

Both.
Depending on their style and the situation of the animation.

What HyMaster said...
Depending on style means how you draw your stickfigure and depending on the situation means using lines in close-up to avoid jagged edges, brush is usually used in drawing a regular sized stick figure, lines can help in stick fig zooms.

brush can be used for movements .
and yeah ... lines are for zooms. but i was wondering .. terk's anim. ( trucido ) was made , all lines . :|

it doesn't matter what tool you use to animate, you can create amazing animations with both, actually you will find out that you will animate way better with the tool YOU prefer, and not what an other random person animate with, i also agree with HyMaster

Ya what he said, just try with both and find out which one you find easier to use