He made his first film, “I Killed My Mother,” a Freudian-tinged, semi-autobiographical film about a gay teenager who clashes with his mother, after he dropped out of college at 17. As one producer after the other rejected it, he scraped together money from his child-acting gigs to make the film himself, at age 20, eventually winning international acclaim. When he later noted wryly that Orson Welles, who made “Citizen Kane” at 25, was a “late bloomer,” the critics pounced, and he turned to Twitter to explain that he had been joking.

“I’m a big mouth. People can dislike me and think I’m a narcissistic brat,” he said. “But one thing no one can ever take away from me is that I’ve always spoken my mind, and I have always been true to myself.”

As we meet, he is the subject of a social media storm over his decision to cut the American star Jessica Chastain from his soon-to-be released film, “The Death and Life of John F. Donovan,” his first English-language film. The movie focuses on an 11-year-old boy and his unlikely correspondence with a closeted actor that is contorted into scandal. (When he was 9 years old, Mr. Dolan wrote a letter to Leonardo DiCaprio, requesting a meeting. He got the address wrong, and it is now framed in Mr. Dolan’s house.)

In the deeply moving film, shot in London, Montreal, New York and Prague, the boy, now a grown man, tells a hard-bitten journalist about how Donovan, torn between fame and personal fulfillment, had changed his life. The original cut of the movie, which stars Kathy Bates, Natalie Portman and Susan Sarandon, and has added cultural resonance in the #MeToo era, was more than four hours long. Eliminating Ms. Chastain, he says, was predicated solely on her character no longer fitting the story line.

“People are saying: How can you cut someone so famous?” he said. “To me, that’s an insult to her talent.”

Born in Montreal, Mr. Dolan was raised by his single mother, Geneviève, a college administrator with Irish roots. He began appearing in commercials for a drugstore chain at the age of 4, and by the time he was 6 he was a child star, appearing in movies, television shows and commercials.