In 1969 Butch Cassidy and the Sundance Kid was a hugely successful film and is still loved to this day.
Paul Newman was a huge star and was the obvious choice to play Butch Cassidy. The director, George Roy Hill, and Paul Newman felt very strongly that this newcomer actor they liked play Sundance. The new guy wasn’t very well known and he’d only made one notable film in which he played a buttoned-down stuffed shirt kinda guy, but they believed that Robert Redford would be perfect to play Sundance. The studio flat out refused. They watched Barefoot in the Park which starred Redford as an uptight attorney and said he was absolutely wrong for the part of Sundance. The battle went on for months and finally Paul Newman stated that if they didn’t cast Robert Redford he was out. Newman was big box office so the studio backed down and cast Redford. The rest is history.