Jason Reitman

Toronto a coming out party for 21-year-old Timothee Chalamet

The kind of breakthrough performance where it's immediately apparent that an actor is going to be a star for years to come is a rarity in movies. Think of Emma Stone in "Easy A" or Jessica Chastain in "The Tree of Life."

But that kind of thunderbolt moment is striking now for 21-year-old Timothee Chalamet, a New Yorker with the talent to speak multiple languages, play numerous instruments and take the festival circuit by storm with a handful of performances — including one assured of ranking among the finest performances of the year.

The Toronto International Film Festival has been a coming out party for Chalamet, who has three films at the festival. He...