John Garfield's violin "performances" are actually played by two professional violinists standing on either side of him, one to bow and one to finger. The actual music was performed by Isaac Stern. In Stern's autobiography, "My First 79 Years" (New York: Knopf, 1999; page 51), when the movie shows closeups of the hands alone playing the violin (without Garfield in the frame), those are Stern's hands.
For the scene where Helen falls off the horse, Joan Crawford performed the stunt herself. Relieved that it had gone well, she nevertheless was forced to do the stunt again when it was decided that Paul (John Garfield)'s rushing over and laying on top of her was too racy. It was reshot, and instead, Helen lies on top of Paul. Crawford later remarked: "I couldn't really understand what was the difference, him on top of me or me on top of him. Well, the difference was I had to fall off the horse again. I did, and I lived to tell the tale."
In 2013 on Piers Morgan Tonight (2011), to promote his autobiography, Robert Blake related that when he was playing John Garfield as a boy in this film, there was a scene he could not get right. Garfield cleared the set and directed Blake himself. After the scene was finished, Garfield told the nine-year-old. "Robert, remember this for the rest of your life. Your life is a rehearsal. Your performance is real."
John Garfield, a method actor, tried to get an emotional bond with the character Joan Crawford played by looking deeply in her eyes. This deeply unnerved Crawford, who told the director: "Tell him to stop looking at me!"