I wrote "female" because the many roles cover a wide age group. We see Virginia Cunningham (Olivia de Havilland) when she was a little girl faced with a family loss that would effect her emotionally. There's a child actress for those flashback scenes. When we get inside the institution with patients and nurses, we see women who are young like newlywed Virginia, middle-aged women and women in their senior years. THE SNAKE PIT hired dozens of talented females for on-camera duty. They weren't all stars but they sure were good. Here's a clip.
20th Century Fox's THE SNAKE PIT brought Olivia de Havilland one of her Best Actress Oscar nominations. Anatole Litvak was a nominee for Best Director and the film was in the Best Picture Oscar category. Her other nominations were in the Best Actress category for Paramount's HOLD BACK THE DAWN (1941) and in the Best Supporting Actress category for the 1939 MGM release, GONE WITH THE WIND.