I was wondering, in your opinion, which actors are most important to their shows, so much that the ratings for said show will fall. Now, I'm not talking about title characters (a la Hugh Laurie on "House") or the star of a show (Tom Welling on "Smallville") since they are their shows and the day they leave will more than likely be a series finale. MY thoughts are more on ensemble shows that studios think they can get away with just moving on to another character...

Here are mine:

William L. Petersen, "CSI" - Rumors have been flying that he could depart at any time... But, they give him more power (executive producer) and money.

Keifer Sutherland, "24" - They might get away with losing Sutherland, but only if they get his replacement in for an entire season to gain some support from viewers.

Vincent D'Onofrio, "Law & Order: CI" - With the show moving to Fridays, it might be over anyways, depending on what kind of drop off it gets.

Mark Harmon, "NCIS" - Don't know if the season 3 finale was his final episode, but I think the show will suffer unless they find a suitable replacement.

Simon Cowell, "American Idol" - As much as people might be annoyed about the guy, they still love to watch him tear into the singers.

Admittedly, it's not called the "The Jack Bauer Show", but Kiefer Sutherland is just as much the star of "24" as Hugh Laurie is of "House" and Tom Welling is of "Smallville".

D'Onofrio is the reason I don't watch CI

Zach Braff on "Scrubs." Every cast member is pure gold, but the central stoylines all revolve around JD.

Steve Carrell for The Office. In a way it is ensemble. But without Michael the show would be nothing.

Zach Braff for Scrubs. Same as Carrell.

Here's one for a SUPPORTING character : Jeremy Piven in Entourage. Without him it's a show about young goof-offs in Hollywood spending money like it's going out of style. Piven gives the show its edge and its focus. Somebody needs to make these kids act like grown ups.. Direct them, motivate them. That's Ari Gold. He makes that show work.

Another easy choice would be Josh Holloway's Sawyer on Lost. Jack was written as the lead, but clearly it's Sawyer's show. He brings the tension AND the comic relief. He was originally the show's villain, but his charisma and intelligence forced the change and evolved the show as we began to see, just as Kate did, that there's more to this guy than he wants us to know. In a show where regulars are killed with alarming frequency, he's one cast member who won't be going anywhere.

oh so easy, Marissa on the O.C.... oops...what?? I guess I didn't hear the gossip.

ok duh. I suppose for me it would have to be, well, that's it. I just wanted to make a joke.

I would vote for Tony Almeda being the lead on '24' if it ever came to that. Whatever season that was that he came out of nowhere to rescue Jack...that was so badass. I miss his wife Michelle tho...raar.

For a past show, Jaleel White (Steve Urkel) on "Family Matters"

Is the Almeida thing a joke too

James Spader - Boston Legal. The show is full of great actors, but he somehow ties it together and makes it special. William Shatner is important too, I just think less vital than Spader.

I haven't watch in awhile...wait for the dvd's. :P

There is no excuse for not watching something as spoiler riddled as THAT show if you really want it to be a surprise when you watch (a pay channel like HBO maybe, but network television?).

Suffice it to say you're comment was its own spoiler.

Since many show are non-ensemble and anchor on one person, I would think a better question - which show survived without the main character/actor leaving?

For that I would vote Valerie Harper for the show Valerie which she left after a contract dispute at the beginning of the 3rd season, and the show became Valerie's Family and then The Hogan Family (they killed her off!).

Hugh Laurie-House

Neil Patrick Harris on How it Met Your Mother, Barney makes the show funny. Ted is probably the most boring character on television

Without him, it would be just an another medical show.

Another one I thought of:

James Caan, "Las Vegas" - Same reason as Petersen and Harmon as none of the other cast members have the same kind of weight.

I'm not sure about Carrell. To me, he is the weakest link in the office. I think he is replaceable.

I think the OP hit on some of the biggest ones, although I can see Mark Harmon as replacable.

I agree with this one... it's probably the only show where the rest of the cast is so much more appealing than the supposed main character.

Victor Garber, to me, was a huge part of Alias. As was Sloane (Ron Rifkin), though as a mostly villainous character, he didn't need to be in every episode.

Smallville wouldn't be the same without Luthor (Michael Rosenbaum)

I'd say the Office would need to keep pretty much all of the main cast... I mean, how could you have the Office without Dwight, for instance? Same with Scrubs, JD is the main character, but the show would lose a ton of it's appeal, to me at least, without McGinley.

somehow I've managed to avoid any spoilers.

I'm still proud of myself for not being spoiled for the Friends series finale which I finally watched a few months ago...I'm invincible!! :P

Although Arrested Development has already been cancelled, Jason Bateman was the sane presence that kept the Bluths together.

I skimmed thru but did anyone say Gandolfini? (sp?)

Sopranos wouldn't be the Sopranos without him.

Edit ta add, I guess Tony is a "title charcter". So...I'll second Steve Carrell.

I'm shocked nobody has named Michael Chiklis. The Shield wouldn't be possible without him.

Going back a while, I think you'd have to say that Larry Hagman was Dallas.

I agree.

While he is a great part of the show, I find that his character becomes tedious at times.

Tony Shalhoub - Monk

What is the show without him?