William Petersen and Marg Helgenberger, <i>CSI</i> William Petersen and Marg Helgenberger, CSI

For eight seasons CSI (returning Thursday, Oct. 9 at 9 pm/ET, CBS) has seen more than its share of stabbings, shootings, stranglings and poisonings. But the real crime is what's about to happen this year: The longtime hit's star, William Petersen, is about to exit the series. Word came this summer that his character, Gil Grissom, will be replaced by a new CSI, played by Laurence Fishburne. As the countdown begins for Petersen's last 10 episodes, we sat down with him and longtime costar Marg Helgenberger (who plays colleague Catherine Willows) to reminisce about their days together and ponder their futures apart.

TV Guide: What would you like to see happen in your last scene together?
William Petersen:
We finally have sex? Believe me, it's been brewing all along. For me, anyway.
Marg Helgenberger: Catherine would love to have a roll in the sack with Grissom!
Petersen: They have this understanding with each other. There's a different kind of respect Grissom has for Catherine that would preclude a relationship on that level. The very thing that was wrong with Grissom and Sara is what he wouldn't allow to happen with him and Catherine.

TV Guide: Maybe after a few eggnogs at the next CSI Christmas party?
That could still be in the offing. Grissom's not going away forever, anyway. Maybe he'll go to Mexico, then come back and take Catherine with him. A tequila holiday!
Helgenberger: [Laughs] Our lost weekend.

TV Guide: Marg, have you thought about a good-bye gift yet?
No! No! No! No gifts!
Helgenberger: I'm still working on it.
Petersen: I'm having a hard time with all this. I may not show up the last day just so I don't have to go through all that.
Helgenberger: Isn't that what John Huston used to do? He'd have his assistant director direct the last scene because he had such a hard time saying good-bye.
Petersen: Maybe I'll have my stand-in do my last scenes.
Helgenberger: I try not to think about that last scene. That's what everyone is feeling right now. We've had a lot of changes in the show these past years, and this is a big one. Everyone feels on unsteady ground right now.
Petersen: We don't talk about it much. Every once in a while, I look across at somebody in a scene and think, "This is weird. This is one of my last times working with you." And then you have to say, "Forget about that."

TV Guide: How will you say farewell?
The hard part is saying good-bye to the cast and crew and writers. It's the people you've been with for nine seasons. It'll be very hard not going in the morning into one of those rooms and seeing them.
Helgenberger: [Laughs] You'll just kind of forget all about this within two days.
Petersen: I promise I'll remember everyone's names.

TV Guide: How will Grissom say good-bye?
We've been mapping out my last 10 episodes that will take Grissom from one place to another. There will be no brain tumor for four episodes. That's the easy way out. We're trying to paint it as we have done with Jorja [Fox] and Gary [Dourdan]. We're trying to make it as real and close to reality as the show has been all along.

TV Guide: Did you guys talk about it before you decided to go?
Marg's known I wanted to go do theater.
I don't know if he said he was moving on in so many words.
Petersen: The first couple of seasons I did. [Laughing, along with Helgenberger] I had the shows marked down on my trailer. I'd "X" them off like a convict.
Helgenberger: There were a couple of seasons there where he was wanting to.
Petersen: The decision was coming over a while. I mean, I wasn't sure I was going to do Season 5. I did the show longer because of everyone I work with, and I'm really proud of what everyone's done. I'm also concerned with the audience. They've been fabulous, stuck with us every year. But at the same time, I knew I needed to go do something. I don't want to get to the point where I don't feel creative anymore just because it's comfortable and fun to hang out with your friends.

TV Guide: You had a pretty decent movie career going before you met Gil Grissom. Do you want to get back to the big screen?
If this was about going off and making films, I'd stay on CSI. I'd love to see us make a CSI film.
Helgenberger: Really?
Petersen: That's the other reason Grissom isn't going to die of a brain tumor.
Helgenberger: I didn't know that.
Petersen: It could be good if we did it right. The whole world is interested in the show.
Helgenberger: I do remember it coming up in years past, but [it] wasn't given a whole lot of validity.
Petersen: You have to make sure it's not just an extended episode or even like the Quentin Tarantino two-parter [the Season 5 ender]. It has to be…
Helgenberger: Edgy!

TV Guide: What will you do in the meantime? Any immediate plans?
The Chicago theater community breathlessly awaits my return. (This fall, Petersen will star in the Steppenwolf Theatre Company's production of Dublin Carol, the play Petersen took time off to star in two seasons ago.)

TV Guide: What about coming back to CSI for an occasional visit?
I'm going to come back and see the guys on set doing episodes I'm not in. And I'm staying on as a producer. The show is important to me, and I'm not abandoning it. I'm coming back on some level. I just don't know what yet.

TV Guide: Do you have a favorite moment together?
I remember a scene in the season finale of Season 3. You were going in for ear surgery.
Petersen: I'm in my little room waiting to go in for this surgery…
Helgenberger : And he's got this little hospital gown on, and I come to say something, and we have this tender moment in the doorway. He starts walking down the hall. And Billy didn't have anything on under the gown. It was a cute, fun moment.
Petersen: And we were going to have the words "The End" up on screen. They shot it but didn't use it. They got afraid.
Helgenberger: They showed my reaction but not your actual…
Petersen: The producers just thought that would make people think of Grissom in a different way. But as long as the characters are going to do what they would do, the audience is with you.

TV Guide: Laurence Fishburne is joining the cast as a new CSI. Will it be an easy transition?
The writers are trying to wrap their minds around Billy having a graceful exit, but also creating a graceful entrance for Laurence.
Petersen: I think it's going be fine creatively for the audience and for the story and for the character. Shooting [the season premiere] was the most difficult nine days I've had in nine seasons on CSI. But Grissom leaving will open windows. Laurence will come in. Catherine could change her position. It will be interesting and different.
Helgenberger: I could be made supervisor, and Laurence is a CSI Level 1. I don't foresee any tension there for the characters, but I haven't had scenes with him yet.

TV Guide: As you count down your time together, what do you think you'll miss most about each other?
The fact that we've done this for nine seasons and still like each other, and still want to work with each other, is an amazing thing. Believe me, for leading men and ladies, that's not always the case. But she is a very fun gal. She's got a great infectious laugh. She gets all my jokes. She's passionate about her work and the show. Many times, she kept me in control. Many times she'd say, "Billy, it's not that bad. Enjoy this. It's the No. 1 show in the world."
Helgenberger: I'm incredibly touched. I know that a part of me is in denial about Billy leaving. I'm crying! I didn't realize how much I've been avoiding facing the reality of CSI without him. And even though we haven't been having as many scenes together as we used to, he's under my skin. I've been very blessed these past eight and a half years not only to have shared in the success of CSI, but to have worked with a man who is an actor's actor minus the pretension, an executive producer minus the superiority, and a stud minus the bulls--t. I love Billy… he's the bomb! I know that we will work together again, and not just as Grissom and Catherine.