straight man

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The partner in a comedy team who feeds lines to the other comedian, who then makes witty replies.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic A member of a team of comic performers who plays a supporting role by helping to set up jokes and punch lines through engaging in preparatory dialog with the principal comedian; a foil who plays such a role in theatrical comedy.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a performer who acts as stooge to a comedian