Warren Beatty was already a star, he’s been in- Shampoo, Bugsy and Reds.
Warren was casted for this role because he saw the script as a producer and gave himself the role because he knew he would be good at the role.
Faye Dunaway was also a star, she’s been in- Network, Super girl and Gia.
i think Faye was casted as Bonnie because of not just because of her natural beauty but because she has this sense of not wanting to follow rules and because she has a sense of badness.
Gene Hackman was in a few movies during this time too- Unforgiven, Superman and No Way Out.
Estelle Parsons had been in a lot of movies too- Watermelon Man, Diane and Two People.