Mousetrap Productions decided to mark the play's 60th year by licensing 60 productions worldwide, and the Walnut, marking its own 203d season, is among the few and the proud. Amy Rodgers, of the Walnut's public relations office, said the cast had been kept in the dark about Waley-Cohen's presence until the last possible minute.
"What kind of reception have you been getting?" Waley-Cohen asked. The actors murmured contentedly, and Bernard Havard, the theater's producing artistic director, said the Jan. 25-March 4 run is poised to break the Walnut record for single-ticket sales.
"We budgeted $230,000," he said, "and now we're near $350,000."
Set in England during the winter of 1952, The Mousetrap concerns a group of strangers trapped at a manor house-turned-inn during a snowstorm. One of them is a murderer, and each is suspect.
Is it one of the newlyweds, or the spinster with the curious background? The architect, the retired army major, or the dapper old man who claims his car overturned in a snowdrift?
What about Professor Plum with the candlestick in the conservatory? Don't even go there.
Dame Christie (1890-1976) said she was inspired to write The Mousetrap by the real-life case of Dennis O'Neill, a boy who died while in the foster care of a Shropshire farmer and his wife in 1945. The play has not been updated over the years - unlike the classic board game Clue, which was reissued in 2008 to entertain the possibility that the murderer might be Cassandra Scarlet (no longer a Miss) with the barbell in the spa.
Nor has Mousetrap ever been filmed. In 1956, British producer John Woolf bought the movie rights on Christie's condition that a film version would not be released until six months after the play closed. Woolf passed away in 1999, and the play - like Queen Elizabeth II, who has been on the throne for as long Mousetrap has been ensconced in London's West End - just keeps going and going.
Christie imposed the caveat largely to keep the surprise ending a secret. A similar request for secrecy is made at the end of each performance to this day. But in this age of Twitter, why do theatergoers join the conspiracy?
"The first time, that speech about keeping the secret was made by Richard Attenborough, now Lord Attenborough, of course," Sir Stephen said, indicating that any request from that esteemed actor would be perceived as an imperative.
Jennie Eisenhower, who plays the female lead at the Walnut, laughed and said she has heard from people who have seen the play multiple times, "and they claim they've forgotten the ending."
"I also think it's because people have enjoyed the play," Waley-Cohen said, "and are willing to play along in keeping the secret."
Contact staff writer Dianna Marder at 215-854-4211, firstname.lastname@example.org,
or @marderd on Twitter.
Read her recent work at http://go.philly.com/diannamarder