EXCLUSIVE: Shirley MacLaine is in negotiations to play the mother of Ben Stiller’s lead character in Fox’s The Secret Life Of Walter Mitty. Stiller also is directing the pic, on which he came aboard after sparking to a new script on the long-in-the-works project from Steve Conrad. Mitty is based on the 1947 Danny Kaye film from the James Thurber short story first published in The New Yorker in 1939, and the story of a perpetual daydreamer had been in development so long that it seemed like it might never be more than a daydream. But now it’s rolling and MacLaine is the first big casting aside from Stiller. Production is set to start in April.