Audrey Hepburn had had a few very minor roles earlier, but she was essentially unknown before being chosen to star in “Roman Holiday”.
Bette Midler in The Rose. I think it was her first film role, and she got nominated for an Oscar. Great death scene.
Keisha Castle-Hughes. With no prior acting experience, Castle-Hughes made her debut in the 2003 independent film, Whale Rider. For her performance, she received an Academy Award nomination for Best Actress at the 76th Academy Awards.
Most of the cast of Bugsy Malone, including the leads Scott Baio and Florrie Dugger.
Marie Dressler had the starring role in her first film Tillie’s Punctured Romance.
Peter Ostrum, the kid from Willy Wonka. His only film appearance. Now works as a veterinarian in rural upstate New York.
Harold Russell in The Best Years Of Our Lives.
He played a disabled World War II vet. He was uniquely qualified for the part as he actually was a disabled World War II vet. Actually, while he was a veteran, I’m not sure if he saw any action … IMDB tells me he was injured (lost both his hands) while training paratroopers.
He won a Best Supporting Actor Oscar for the role.
Didn’t Barbra Streisand win an Oscar for Funny Girl, her first film?
He was my first thought, too. He was pretty good, too. He came off as sweet and unassuming, much like as Charlie was meant to be. But he also wasn’t an obnoxiously cute type. Good all around.
As I understand, the Superman roles were cast with the intention of getting an unknown. AFAICT from imdb, Christopher Reeve had only done a couple of TV shows before being cast. Brandon Routh appears to have done several more, but it still looks like just TV shows.
I also came in here to say this.
Robert Di Niro had a single bit part before being cast as one of the leads in Greetings.
Julie Andrews’ first role was Mary Poppins.
He was working with explosives for a training film when a faulty fuze caused an explosion.
I think “debut” means first time ever acting professionally! By that definition, Jennifer Hudson pulled it off by starring and winning an Oscar in Dreamgirls.
Not true - IIRC her first role was in a small indie comedy film (its late and the name escapes me) that college buddy Dan Rosen directed.
Missed edit window: oops, I was wrong, Mask was first, The Last Supper 2nd: Cameron Diaz - IMDb
Glenn Close’s film debut as Jenny Fields in The World According to Garp won her an Oscar nomination.
Nobody has mentioned Robert Duvall as Boo Radley in To Kill A Mockingbird.
How about Anna Paquin? She won an Oscar for The Piano.
Not really a starring role. A crucial role, yes, but not starring.
Two of the Harry Potter kids (Rupert Grint and Emma Watson) appear to have begun professional acting with the Harry Potter series (from what I can gather from IMDB).
Daniel Radcliffe did a bit of TV before becoming Harry Potter, it looks like.
Kate Winslet had a starring role in Heavenly Creatures, her film debut.