Twentieth Century Fox has set Keanu Reeves to star in “The Day the Earth Stood Still,” its re-imagining of the 1951 Robert Wise-directed sci-fi classic. Reeves committed over the weekend to play Klaatu, a humanoid alien who arrives on Earth accompanied by an indestructible, heavily armed robot and a warning to world leaders that their continued aggression will lead to annihilation by species watching from afar. Erwin Stoff is producing, with Scott Derrickson (“The Exorcism of Emily Rose”) directing from a script by David Scarpa. Reeves’ commitment puts the picture on track for a late fall or early 2008 production start. Studio sees it as a tentpole.