When the miners greet their women by putting their earnings in baskets, actress 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. Director 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.
Two major factors entered into the decision to shoot the film in Southern California: (1) the continuous bombing of Britain by the Nazis; (2) the nervousness of Fox executives about the film's pro-union storyline. These factors, and William Wyler's reputation for perfectionism, swayed Fox to keep the filming done in the U.S.
Plans to film in Wales were abandoned due to WWII; an 80-acre set was built in the Santa Monica Mountains at Brent's Crags, near Malibu. The design of the village was based on the real Cerrig Ceinnen and nearby Clyddach-cum Tawe in Wales.
John Ford became the first director to win three Academy Awards for Best Director, having previously won for The Informer (1935) and The Grapes of Wrath (1940). 11 years later he would win his fourth for The Quiet Man (1952), a record unmatched by any other director.
Roddy McDowall had only been in America for two weeks before being cast in the leading role of Huw. He had been evacuated from Great Britain with his mother and sister to keep out of harm's way of Nazi bombardments of the islands.
Sara Allgood clashed repeatedly with John Ford during the making of the film, something the director found rather unsettling as he wasn't used to having his authority questioned. Nevertheless her performance was recognized by the Academy with a Best Supporting Actress nomination.
The author of the novel, Richard Llewellyn had claimed to have based the book on his own knowledge of the Gilfach Goch area, but this was proven false, as Llewellyn was English-born and spent little time in Wales. As it turned out, he had actually gathered his facts from conversations with local mining families.
"The Screen Guild Theater" broadcast a 30 minute radio adaptation of the movie on March 22, 1942 with Sara Allgood, Donald Crisp, 'Roddy McDowall', Maureen O'Hara and Walter Pidgeon reprising their film roles performing with Rhys Williams. This was followed by a 60-minute performance on September 21, 1942, with all the same except for Williams. Years later, Crisp repeated his role in radio productions of the same on March 31, 1947 and September 28, 1954.
Although Alfred Newman's justifiably highly-praised score utilized many Welsh melodies as well as the services of virtually every Welsh singer then living in Southern California, his principle love theme was based on the traditional Irish folk song "The Sixpence," not on anything Welsh.
In 1966 a musical version of the movie entitled "A Time for Singing," was produced by Alexander H. Cohen with music by Mel Brooks collaborator John Morris, lyrics by Gerald Freedman and John Morris, and a book by Freedman and Morris. Although Cohen was enthusiastic about the project, calling it the best musical he had ever produced, it closed after 41 performances. Bing Crosby did record two of the songs for Reprise records.
The mining village set cost $110,000 to construct and was modelled on the towns of Cerrig Ceinnen and Clyddach-cum Tawe. The studio brought in blocks of coal weighing over a ton apiece for the construction of the mines. To create the impression that coal slag covered the landscape in the opening and closing scenes, John Ford had the hillside painted black.
Fox technicians turned an 80-acre plot in Brent's Crags, near Malibu, into the Welsh mining village of the story. By that time, the budget had been scaled down to $1.25 million, which didn't allow for any shooting on actual locations. With the start of World War II in Europe, shooting overseas was almost impossible for the Hollywood studios anyway. Darryl F. Zanuck also abandoned his plans to shoot in Technicolor, partly because the Malibu countryside would not match the colours of Wales.
Historians have called the way the wind plays with O'Hara's veil when she leaves the church after her wedding a stroke of luck for John Ford. Far from it, he had instructed the crew to set up wind machines to fan the veil into a perfect circle behind her head then blow it straight up into the air.
For the most part, the atmosphere on the set was totally congenial, with John Ford trusting his cast to deliver strong performances with a minimum of guidance. The one problem for Maureen O'Hara occurred when she pointed out - in front of the cast and crew - that the basket with which she and Sara Allgood were supposed to receive Donald Crisp's weekly wages was not of the period. In response to what he considered her breach of etiquette, he removed her from the scene. An hour later, an assistant called her back to the set and handed her a new, historically accurate basket, so Ford could shoot the scene with her in it.
Sara Allgood was the only actor who gave John Ford any trouble. At one point, she complained that a scene they were about to shoot wouldn't play. Ford called writer Philip Dunne to the set and relayed her opinion to him. Having worked with Ford before, Dunne knew what to do. He ripped the scene out of the script and said, "Now it plays!" Then Ford turned to Allgood and said, "The sonofabitching writer won't do anything to help us, so we'll have to shoot it the way he wrote it."
Initially, John Ford had Rhys Williams record the narration as the adult Huw Morgan. When he became concerned that audiences would recognize the voice as belonging to Williams's on-screen character, the boxer Dai Bando, he had actor-director Irving Pichel re-record the lines, which had to be read in the same rhythms as Williams's to match sequences already cut to the narration. At one point, British prints of the film featured Williams's voiceovers.
The trivia items below may give away important plot points.
The title of the novel appears in two sentences. It is first used in Chapter XXX, after Huw has just had his first sexual experience. He sits up to "look down in the valley." He then reflects: "How green was my Valley that day, too, green and bright in the sun." Needless to say, this sexual experience was cut out of the film. The phrase is used again in the novel's last sentence: "How green was my Valley then, and the Valley of them that have gone."
The author continued the story about Huw Morgan's life in 3 sequels. 'Up into the Singing Mountain' (1960) in which Huw emigrates to Argentina; 'Down Where the Moon is Small' (1966), Huw's life in Welsh-speaking parts of Argentina; and 'Green, Green My Valley Now' (1975), in which Huw returns to Wales. None of these have been made into films, and 'How Green Was My Valley' is still the most consistently popular novel of the series.