How do I document an interview in MLA style?

Create a works-cited-list entry for an interview as you would for any other source: follow the MLA format template. In general, treat the person being interviewed as the author. Then provide the title of the interview:

Saro-Wiwa, Ken. “English Is the Hero.” No Condition Is Permanent: Nigerian Writing and the Struggle for Democracy, edited by Holger Ehling and Claus-Peter Holste-von Mutius, Rodopi, 2001, pp. 13–19.

If the interview is contained in another work, the interviewer’s name may be included in the optional-element slot after the title of the interview and followed by a period: 

Saro-Wiwa, Ken. “English Is the Hero.” Interview conducted by Diri I. Teilanyo. No Condition Is Permanent: Nigerian Writing and the Struggle for Democracy, edited by Holger Ehling and Claus-Peter Holste-von Mutius, Rodopi, 2001, pp. 13–19.

If the interview is untitled, follow the guidelines on pages 28–29 of the MLA Handbook and include the generic description interview. If the interview is not contained in another work, you may list the interviewer’s name in the “Other contributors” slot after the description and follow it with a comma:

Walcott, Derek. Interview. Conducted by Susan Lang, 22 Oct. 2002.