The charity initiative, which brings celebrities and playwrights together in an all-day theatrical marathon at the Broad Theater, sees writers, directors and actors meet to plan their ideas for the plays, which are then performed live just 24 hours later.
The event has been a fixture on Broadway for years and Saturday's show marked the first time it had hit the west coast of America - and the stars taking part ensured it was one of the wackiest yet.
Hitch actress Mendes proved her comedic capabilities by pulling on a pumpkin costume for her skit, while Welsh hunk Gruffud went even further, stripping down to a women's bikini and performing a dance in a sketch about sex therapy.
Other celebrities performing included Dr Who star Matt Smith, who is currently trying to crack America. Smith played Batman in his play, alongside fellow Brit Saffron Burrows, while his model girlfriend Daisy Lowe looked on from the audience.