Joseph McBride, Searching for John Ford wrote:
Ford was not the first director assigned to the film by Darryl F.Zanuck. William Wyler spent three months preparing the picture. he cast many of the parts, oversaw the construction of the sets designed by Richard Day and nathan Juran, and spent ten weeks working on the script with Phillip Dunne. in Dunne’s view Ford made little contribution to the script beyond adding some bits of business and lines of dialogue… But while others were primarily involved for shaping the adaptation of the novel before Ford was assigned to the project, Dunne nevertheless acknowledged to me that Ford “did what any good director does — he made it his picture while shooting it.”
Jealous of the blockbuster success of David O.Selznick’s Gone with the wind, Zanuck initially envisioned How Green Was My Valley as his answer to Selznick on a similar grandiose scale. It was to be a four-hur Technicolorerpic shot on location in Wales, starring Tyrone Power as the grown up Huw Morgan, Laurence Olivier as the preacher Mr Gruffydd, Katherine Hepburn as Angharad, and Greer Garson as Bronwen. Zanuck rejected scripts by Liam O’Flaherty and Screen Writer’s Guild president Ernest Pascal because he thought they put too much emphasis on the stuggle of unionization of the coal mine in the village.
Even after the political issues of the screenplay were settled, Zanuck and Dunne still had to solve the problem of running time. The Fox front office in New York was becoming uneasy with the costly project, so Zanuck decided that the script should be cut in half to a conventional two hour length and that the film should be made in black and white. because of the outbreak of war in Europe, location shooting was out of the question, so the spectacular set of the winding street of homes leading up to the mine was constructed on the Ford ranch in the Malibu hills.
* Audio Commentary by Anna Lee Nathan and Joseph McBride
* AMC Backstory: How Green was my Valley