THE sight of an actor lost in thought, murmuring lines en route to an audition, is hardly uncommon in a city’s theater district. But passengers on an Amtrak train from Philadelphia to New York one morning two years ago may have noticed something far rarer: a performer prepping a bit of silent-film high jinks.

Rob McClure was on his way to his sixth callback for the lead role in a new musical about Charlie Chaplin, and he’d been asked the day before to come back with “a two-minute Chaplin-y thing,” he recalled recently over dinner.

Previous auditions had focused on trying out songs and scenes from the show, then called “Limelight” and headed for La Jolla Playhouse in California. But before building a large-cast, big-budget musical around a relative unknown, the show’s director, Michael Unger, wanted another piece of insurance: a “flash of Chaplin genius,” as Mr. McClure put it.

“It was 2:30 in the morning the night before, and I could not think of anything,” said Mr. McClure, who would eventually land the part in the La Jolla production and also play it in the retitled “Chaplin,” now in previews on Broadway, and scheduled to open at the Barrymore Theater on Sept. 10. “Finally my wife said: ‘Why don’t you bring music? That way you won’t be hung out to dry with silence.’ ”