If you are referring to the printed publication/script of a play, emphasis is given to the author first and the play is referenced as a book (see example):   

Format:  
FAMILY/SURNAME, Initials. (Year of publication – in brackets). Title of Play- in italics. Place of Publication: Publisher.   

Bibliography example:  
OSBORNE, J. (1957). Look Back in Anger. London: Faber and Faber.   

In-text citation example:  
(Osborne, 1957). 
…supported by Osborne’s (1957) play Look Back in Anger… 
‘Direct quotations are placed in single quotations marks’ (playwrights’ Surname, followed by Year of Publication, Act, Scene and/or page number – in brackets).

If you refer to production and performance elements of a play that you have viewed within your assignment, your reference will need to give emphasis to the title of the play first (see examples).   

Format:  
Title of Play- in italics. By AUTHOR. (Year of publication – in brackets). [Play – in square brackets]. Production date. Theatre and Place of performance. Performer’s names. Director.   

Bibliography example:  
A Round of Applause. By HARRY GREEN. (2006). [Play]. First Produced 18th June. Round Theatre, Burns Wood. Performers: I. Bloggs & S. Jones. Director: Harry Green.