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Reminds me of a question I had for theatre mavens. I saw Spamalot in London last week. In the program, in addition to the main cast, they also had "The Ensemble," and "Swings." So what's the difference? The understudies were sometimes listed in "Ensemble" and sometimes in "Swings." The Dance and Fight Captains (whatever that means?) were both swings.
The ensemble is members of the cast who play more than one small role, or dance in the background. They perform every night, in set roles.
An understudy studies a particular lead or supporting role, and will go on in certain circumstances. They can be a member of the cast, or not (I have a friend who understudies the main role in Hairspray; she does not perform daily).
A swing is an understudy to members of the ensemble. They study numerous roles of people in the ensemble, and will generally go on when someone who is an understudy in the ensemble gets bumped up to the role they are understudying. They "swing" from role to role.
Oh, and the Dance Captain in a show is someone who can uphold the choreographer's vision (since a choreographer might choreograph a show and then not come back for a long time), make sure everyone knows their steps, rehearses with new people, etc.