CSI: CRIME SCENE INVESTIGATION. 9 tonight, Channel 3.

FANS OF CBS' "CSI: Crime Scene Investigation" have had months to get used to the idea that Gil Grissom (William Petersen) would be leaving.

That's how it is in television, where popular characters aren't generally allowed sudden moves, lest a promotional opportunity be wasted.

Inside the box, though, the characters themselves don't get the press releases. So when Grissom tonight lets his office family know that he'll be moving on, it comes as a bit of a surprise to most of them.

Not one, however, that's allowed to overwhelm this week's case, which involves a serial killer or two and the introduction of Laurence Fishburne as Dr. Raymond Langston, a physician and criminology professor who we know (and the CSIs don't) will eventually be joining their twisted little group.

There were suggestions last summer, when Fishburne's casting was first announced, that his character might be a little more than average twisted, but there's no sign yet that he's going to turn out, for instance, to be the "Dexter" sort of crime-scene investigator, one whose interest in homicide might be a little too personal.

That's all to the good, the strength of "CSI" lying in part in its willingness to let characters develop slowly, leaving viewers to sift the evidence that may not always be left in plain sight.

For the most part, this is still Grissom's night - though not his last on the show, the actual departure being scheduled for Jan. 15 - and the way it's handled may have the ring of familiarity to many Americans, for whom workplace departures have become a bit too common lately.

Not that Grissom's being laid off or bought out or any of the other phrases we've learned to apply either to ourselves or to the people who aren't there anymore.

The guy who plays him wants to move on, and manages to convince us that Grissom shares his feelings (if not necessarily his interest in doing more theater).

If the people who work with him are a bit taken aback, they at least are not stunned into the kind of stupidity that so often overtakes their ABC counterparts on "Grey's Anatomy."

There's always work to do on "CSI," and Petersen or no, that work will go on.

Mellencamp film debuts

"Homeward Bound: John Mellencamp," a new documentary from Philadelphia-based Stage 3 Productions (a subsidiary of Center City Film and Video), premieres at 9 tonight on the Biography Channel.

The film, which traces the singer-songwriter's career from its roots in Seymour, Ind., the small town where he was born, with spina bifida, in 1951, boasts extensive interviews with Mellencamp and with those who know him, including former band members who chose a different path from the guy once known - to his embarrassment and theirs - as "Johnny Cougar."

There's not a lot of personal dishing of the "E! True Hollywood Story" sort, but "Homeward Bound" does a nice job of showing how record companies shape our perception of their artists, and what it took Mellencamp to get that Cougar off his back.

In any case, Mellencamp fans won't want to miss the included concert, filmed at an Indiana theater where he'd performed 30 years earlier. *

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