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Henry Fonda kept the hat he wore in the movie for the rest of his life. Before he passed away in 1982, he gave it to his old friend Jane Withers. Apparently he and Withers, when she was an 8 year old girl and he a young man, did a play together before Fonda made movies. Fonda was so nervous to go onstage that little Jane took his hand, said a little prayer to ease his nerves, and the two of them became good friends for life.
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 Ford banned all makeup and perfume from the set on the grounds that it was not in keeping with the tone of the picture.
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.
While filming the Joads' car traveling down the highway, John Ford wanted to add a shot showing the large number of caravans heading west, so the film's business manager stopped actual cars making the trek and paid the drivers five dollars to escort the Joads' jalopy for the cameras.