When Tom Cruise pulls a real-life wig-out — jumping all over Oprah’s couch, say — it’s creepy.

But when he yanks out the stops on sanity on behalf of a movie … well, then it’s pretty darn entertaining. Wish he’d get crazy more often.

For “Knight and Day,” Cruise cannily melds his leading man charisma with his late-blooming talent for character roles (“Magnolia,” “Tropic Thunder”), turning himself into the live-action version of a “Road Runner” cartoon. For a while it’s giddily liberating.

In James Mangold’s action comedy, Cruise plays Roy Miller, a self-assured charmer with a glint of madness in his eyes. This government-trained killer can fly all sorts of aircraft, dodge bullets and leap from one vehicle to another at highway speeds, all the while carrying on a calm, dispassionate conversation as if nothing at all remarkable was happening.

We see Roy through the eyes of single gal June Havens (an amusingly panicked Cameron Diaz), who meets him at the Wichita airport while both are waiting to board a flight to Boston. (A direct flight from Wichita? Yeah, sure.)

June is so impressed that while in the plane’s restroom she fantasizes that he might be the One. She emerges to find Roy the sole survivor in a cabin full of dead people and herself in the middle of a worldwide pursuit.

Roy has stolen the prototype of a battery so awesome that it can power a good-sized city indefinitely. His CIA colleagues (Peter Sarsgaard, Viola Davis) claim he’s gone rogue; Roy says he swiped the item only to keep it out of the hands of turncoat agents.

June doesn’t know who to believe, and when she gets too worked up about the mayhem around her, Roy apologetically pumps her full of sedatives. There’s a nifty sequence that has June awakening from her torpor just long enough to realize she and Roy have tumbled out the open door of a small aircraft. Then she blacks out again, only to find herself in a red bikini on a tropical beach while Roy prepares fresh fish over a campfire.

“Knight and Day” cannot sustain its manic energy, particularly after we finally figure out who’s who. But for the first 40 minutes, when we’re not sure if Roy is totally nuts or on the level, it’s lightweight but enjoyable summer fun.

Film focus

WHAT: “Knight and Day”

RATED: PG-13 for sequences of action violence throughout and brief strong language

RATING: 2 1/2 stars

RUNNING TIME: 1 hour, 50 minutes