character animation in flash is possible but very time consuming. animating body parts is tricky and near impossible to get perfect with just 1 man on the show. but its fun to do and its very satisfying when you get everything looking fluent.
I work in a cartoon studio, and at any time we have 30+ animators working out the finer points of moving the characters. So i can only imagine that something as high budget as futurama has alot more people working on it.
if you want to learn the basics of character animation in flash. there is lots of reading out there to be found.
heres a cool flash based tut to get you started.
http://www.toddgallina.com/how/2.html