CSI: CRIME SCENE INVESTIGATION.

9 tonight, Channel 3.

UNIVERSAL CITY, Calif. - William Petersen is really, truly gone from CBS' "CSI: Crime Scene Investigation."

He's not in the room that used to be Gil Grissom's office.

Not in the DNA lab, not in the autopsy room, not in the ballistics area, or in any of the other dimly lighted rooms where you might once have expected to see him.

He's not, in fact, anywhere to be found on the show's Universal Studios set, which reporters toured earlier this week after interrogating Petersen's former colleagues about his final days there, about the likelihood that he'd return from time to time ("Billy said he'd be back to make sure I didn't burn the joint down," quipped the show's newest star, Laurence Fishburne), about whether the show itself, still television's most-watched scripted show in its ninth season, had an exit strategy, too ("We're not quitters on 'CSI,' " said executive producer Carol Mendelsohn).

Petersen, who'll remain in the show's credits as an executive producer, makes his official goodbyes tonight in the conclusion of a two-parter involving serial killers, a far from endangered species on CBS, which relies on them almost as much as it has on the man whose portrayal of a dogged, introspective scientist helped put "CSI" on top.

You won't, as the promo writers like to say, want to miss the last five minutes.

Many faces of 'Tara'

Showtime has its sympathetic serial killer ("Dexter"), its not-so-sympathetic king ("The Tudors") and its sex addict with writer's block ("Californication"), but its most intriguing character yet might be Tara Gregson (Toni Collette), a Kansas homemaker and artist who suffers from multiple personalities, or, as it's known in the medical literature, dissociative identity disorder.

On Sunday, the premium cable network unveils Tara and some of the other people who live inside her in the 10 p.m. premiere of "The United States of Tara," a sometimes funny, sometimes heartbreaking half-hour about a family dealing with mental illness as best they can.

"It was not a flight of fancy in any way," Oscar-winning writer Diablo Cody ("Juno"), who created the characters from an idea by executive producer Steven Spielberg, told reporters yesterday, insisting that cases like Tara's exist. "I wanted to show how a normal, affectionate family copes with something like this," she said.

Cody's gift is for characters who do and say the unexpected while remaining real, but without Colette ("Little Miss Sunshine") , it's easy to imagine "Tara" as a train wreck, or, worse, an acting exercise. Somehow she imbues Tara's alternate personalities - known as the "alters" - with enough substance to make them interesting, without making them so real that we forget they're a manifestation of an illness.

The actress, who confessed she'd "never had any ambitions to work in TV," said that after reading the pilot script, she'd turned to her husband and said, "I have to do this."

Though it's not entirely clear how many personalities live inside Colette's character, the first four episodes introduce three "alters": "T," a 16-year-old who goes shopping with Tara's teenage daughter (Brie Larson); "Alice," a June Cleaver-like figure who has a special bond with her 14-year-old son (Keir Gilchrist); and "Buck," a gun-toting, porn-loving male whom John Corbett, who plays Tara's extremely understanding husband, declared his favorite.

Colette, for her part, won't say. "I think it would be like a bad parent choosing a favorite kid."

DVR alerts galore

"Tara" is part of a night of Showtime premieres on Sunday, with "The L Word" returning for its sixth and final season at 9 p.m. and "Secret Diary of a Call Girl" launching its second season at 10:30. Some other premieres of note:

_ "Friday Night Lights" (9 p.m. tomorrow, Channel 10). One of television's best shows has been the exclusive province of DirecTV's 101 Network for months now, but finally "Friday Night Lights" is returning to NBC, with a third season that feels more like the first. In other words, no homicides, accidental or otherwise, just the very real human drama of life in a Texas town where football touches nearly everyone's lives.

You don't, however, have to give a hoot about high school football (I know I don't) to care deeply about the people of Dillon.

_ "Battlestar Galactica" (10 p.m. tomorrow, Sci Fi). "Galactica" resumes its final season with the wandering humans and Cylons reeling from the discovery that the Earth they'd been searching for is a dead planet. If not exactly the feel-good episode of any season, it's nevertheless a deeply moving one.

_ "Big Love" (9 p.m. Sunday, HBO) is back after a very long hiatus with a third season that finds the polygamous Henrickson clan as close, and as confused, as ever as Bill (Bill Paxton) pursues a deal he hopes will give them some security and wives Barb, Nicki and Margene (Jeanne Tripplehorn, Chloe Sevigny, Ginnifer Goodwin) look for security in other ways. Also back on HBO, at 10 p.m. Sunday: The hilarious musical duo of "Flight of the Conchords." *

Ellen Gray is covering the Television Critics Association's winter meetings. Read more at go.philly.com/ellengray, chat with her at 4 p.m. today on philly.com or send e-mail to graye@phillynews.com.