California Doubling: The film was originally intended to be made in the UK, but World War II made that idea all but impossible, so it was filmed on the 20th Century Fox backlot. According to movie lore, the move to California meant that the movie would have to be filmed in black and white, rather than color as originally intended, because the colors of the flowers and other vegetation in California would not match the colors in Wales.
Hide Your Pregnancy: Anna Lee discovered she was pregnant halfway through filming and made a point of not telling John Ford about her condition for fear of losing her role.
Throw It In!: Cyfartha's final line, "'Tis a coward I am, but I will hold your coat," was added by John Ford over the objections of screenwriter Philip Dunne.
Wag the Director: When the miners greet their women by putting their earnings in baskets, Maureen O'Hara stopped the scene's filming once she noticed that her basket was a modern Kraft basket and not a basket of the movie's period. John Ford was so upset by being corrected in front of the cast and crew that he closed down the set and told O'Hara to wait on a nearby hill until he called for her. Fuming, O'Hara waited an hour before an assistant came to retrieve her but was satisfied to see that the basket had been changed upon her return.
It was originally envisioned as a sprawling three-hour epic filmed in Technicolor and on location in the UK, with Tyrone Power as an adult Huw. World War II put the the damper on those plans, so they were scaled back to be more modest and focus on Huw's childhood instead of his journey to adulthood. The BBC would later adapt the whole novel in miniseries format.
William Wyler was originally set to direct. He left the production to make The Little Foxes.