Robin Williams by Vera Anderson/WireImage.com Robin Williams by Vera Anderson/WireImage.com

I told you Law & Order: SVU's 200th episode was going to be big, and now I've got proof: Sources confirm to me exclusively that Robin Williams has inked a deal to guest-star in the landmark episode on April 29. The Oscar winner will play a Svengali-like figure who tests Benson and Stabler like never before.

Exec producer Neal Baer says the episode is "one of the best cat-and-mouse stories we've ever done," and describes Williams' character as "a guy who has a problem with authority figures." Adds Mariska Hargitay: "He is one of these charismatic human beings that has uncanny power over people to get them to do what he wants."

Williams' casting comes just hours after it was announced that another movie star, Bill Pullman, would be doing an arc as a love interest for Benson. But that's only half the story. For the full story, check out Ask Ausiello tomorrow. - Additional reporting by Ileane Rudolph