It's wrong to tell a joke that hurts someone.
James felt jealous of Don because Don got the lead role in the community play while James was just an extra.
James wants to feel better about not getting a speaking role.
James complains to anyone who will listen that he deserves a speaking roles.
James begins making jokes about Don's height, weight, and vocal mannerisms.
Several other actors in the play sympathize with James because the group is fairly tight-knit.
Don eventually gets tired of the jokes and asks the director to make James stop or kick him out of the play.