NEW YORK — Kevin Kline proved the able mystery guest for the March 30 opening perf of “The Play What I Wrote,” starring Hamish McColl, Sean Foley and Toby Jones. Director Kenneth Branagh calls the show’s casting gimmick, with a new guest every few days, a real high-wire act.“There’s no understudy,” he claims. “As long as I’m in town, I’m the fallback.” Maybe Branagh could get some of his Brit friends in attendance to oblige: Michael Palin, Alan Cumming, Roger Moore and Eddie Izzard all turned out. Producer Mike Nichols also might be able to help. His contingent included Natalie Portman, Elaine May, Nathan Lane, Billy Crystal and Christine Baranski.