Answer:
That is the ball and socket joint
Viola's disguise as Cesario creates many humorous situations. The audience can laugh at Olivia for falling so deeply in love with Cesario because they know it is actually Viola and they can find humor in how surprised Viola is to discover Olivia's love. Also, Sir Andrew and Sir Toby try to engage Cesario in a fight. In some scenes they mistake Sebastian for Cesario, and this is part of the humor, because when Cesario is actually threatened with a fight the fact that she is a girl and so scared of a fight would be funny.
Viola's disguise also creates much of the conflict too, because her presence stirs up anger in Sir Toby and Sir Andrew (not wanting another boy to have Olivia's attention). Viola also cannot express her love for Orsino because of her disguise, which causes tension and conflict, especially at the end of the play. When Sebastian enters the action and the two become consistently confused for each other, both humor and conflict arise.
In time, of course, all issues get resolved, but not necessarily in the way you would like.
