Prior to filming, producer Darryl F. Zanuck sent undercover investigators out to the migrant camps to see if John Steinbeck had exaggerated about the squalor and unfair treatment meted out there. He was horrified to discover that Steinbeck had actually downplayed what went on in the camps.
John Steinbeck loved the movie and said that Henry Fonda as Tom Joad made him "believe my own words".
John Steinbeck was particularly enamored with the performance of Henry Fonda as Tom Joad, feeling that he perfectly encapsulated everything he wanted to convey with this character. The two became good friends. Indeed Fonda did a reading at Steinbeck's funeral.
John Ford banned all makeup and perfume from the set on the grounds that it was not in keeping with the tone of the picture.
According to Henry Fonda, John Ford preferred only one take and little or no rehearsal to catch the most spontaneous moment. For the key climactic final scene between Tom and Ma, Ford didn't even watch the rehearsal. When the time came to shoot, Ford led Fonda and Jane Darwell through the silent action of the scene, preventing them from starting their lines until the two actors were completely in the moment. It was done in a single take and Fonda said on screen it was "brilliant."