Julia Roberts did study acting.

Jimmy Stewart, Bette Davis, Katharine Hepburn, Tilda Swinton, Humphrey Bogart, Jack Nicholson, Jean Gabin, Sidney Poitier, Anthony Quinn, Amy Adams, and Tommy Jones all had apprenticeships as repertory players in theater companies.

Most of the rest were child stars or had family connections to industry.

Renee Zellweger (who acted in college) and Jonny Depp found their way to it.