Slater appeared in Tuesday night's public dress rehearsal of "One Flew Over the Cuckoo's Nest," the BBC reported Wednesday.
Later, Slater thanked the audience for being "fantastically supportive".
The play's opening was delayed four nights after Slater, 34, became ill during rehearsals.
The play, adapted from the book by Ken Kesey, has been beset with problems, including director Guy Masterson quitting weeks before the scheduled opening night.
Masterson blamed stress for his decision to leave the production. He was replaced by co-directors Tamara Harvey and playwright Terry Johnson.