The acclaimed filmmaker, who eventually cast Josh Brolin as the US president, told GQ that he had gone as far as conducting make-up tests on Bale to make him look older.
"I needed a star," Stone commented, "and Josh Brolin was not a star. Originally I went for Christian Bale. We did some rigorous prosthetic tests and spent a lot of dough - thousands and thousands of dollars - and then Christian said, 'I just don't feel like I can do it.'
"I met Josh and liked him. He was more rural Americana. But man, he was scared s**tless."
W will premiere in the UK at the BFI London Film Festival next month then open in cinemas on November 7.