Liev Schreiber, <EM>CSI</EM> Liev Schreiber, CSI

It's the sort of mystery that would make even Gil Grissom scratch his head. The lead actor in one of the most popular shows on television disappears mid-season and is willingly replaced by a Tony-winning hotshot who supposedly "doesn't do TV."

Care to give an explanation, bub?

"Would you believe it just sounded like a neat idea?" Liev Schreiber says, though he knows he has to do better than that.

After all, CBS' CSI: Crime Scene Investigation (Thursdays at 9 pm/ET) is among the longest-running, most obsessed-over prime-time dramas going, largely because of its beloved investigator in chief, played by William Petersen. You don't just do away with Grissom without raising a few million eyebrows.

Call it a case of success run amok. After more than 100 episodes, Petersen has admitted he needs a break, which he's taking so that he can appear in a play in Rhode Island. (He'll return to CSI in February.) As exec producer Carol Mendelsohn says, "Nobody expected the show to run as long as it has." And while Petersen's character is "on sabbatical" from the lab (Grissom will be teaching at Williams College while he sorts out his own job frustrations), a seasoned CSI from Baltimore named Mike Keppler will fill in.

The stand-in role, which kicks off this week, marks the television series debut for Schreiber, who's best known for distinguished performances on Broadway (Glengarry Glen Ross) and in films (The Manchurian Candidate). Lately, he's also known as the boyfriend (rumor says fiancé) of gorgeous Aussie actress Naomi Watts — but let's leave that to the tabloids.

So why CSI? "I've always been scared of TV," Schreiber says, and for reasons similar to those that Petersen has cited. The week-in, week-out schedule can be a grind, and actors are sometimes pigeonholed by trademark roles. But after a meeting last summer with Mendelsohn, Schreiber was hooked. "The show is so cool, and there are so many places to take these characters," he says. "You've got death, mysteries and all that creepy stuff. What's not to like?"

The gig, which will run for at least three episodes, has been a humbling one for Schreiber. "You learn quickly that the real stars of this show are the dead bodies and the science," he says. "The props are way more important than you."

And if nothing else, he's getting a crash course in ickiness. "There's an episode coming up where a guy's bones and veins were stolen by a company that resells body parts," he says. "I have to reach into a cadaver's leg and pull out an umbrella and some PVC pipe some crazy old mortician used to replace the femur. I assure you, no amount of actorly training can prepare you for situations like that."

Perhaps, but we're predicting a first-rate actor like Schreiber will start pulling out Emmys if he sticks around TV.

For some CSI casting news, plus some Numbers scoop, pick up the Jan. 22 issue of TV Guide, featuring American Idol on the cover.

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