How's this for a weird fact? Actor Robert Redford (who plays Johnny) never actually saw this movie until 2004. He must have had better things to do than watch himself act. (Source)

Apparently, Jack Nicholson was the first choice to play Johnny Hooker before Redford decided to step up and play the role. (Source)

You know how Doyle Lonnegan walks with a limp? Well it turns out that the injury was real. Just a week before filming started, actor Robert Shaw slipped on a wet handball court and tore up the ligaments in his knee. But like a total pro, he rolled with it and made the limp part of his character. (Source)