Many moviegoers will head to their local multiplex to see Tyler Perry in "Alex Cross" this weekend. What they might not realize is that the psychopathic killer trying to kill the title character is played by none other than Matthew Fox.

Best known for his role as Jack Shepard in the iconic TV series "Lost," Fox underwent a five-month transformation to play the crazy Picasso in "Cross."  Sitting down with Fox, you'd think he'd be tired of providing details on how he lost a staggering 40 pounds. Instead, the 46-year-old - who looks much closer to his old self nowadays - went into staggering detail on how he pulled it off. And no, it wasn't P90X.

For any fan of Fox's or if you're wondering how an actor can accomplish such staggering physical changes in such a short amount of time, watch the interview embedded at the top of this article.

Fox will next be seen opposite Tommy Lee Jones in "Emperor" and in the long-delayed "World War Z" next summer.

"Alex Cross" is now playing nationwide.