Tina Fey and Oprah Winfrey in the "Believe in the Stars" episode of 30 Rock.

Just say no, Tina Fey.

We know all your celebrity pals want to be on your critically acclaimed show, 30 Rock.

It's the cool place to be. It's kind of like when all those guest stars lined up to be villains on Batman in the 1960s.

But here's the truth about 30 Rock, which was created by Fey and airs Thursdays on Citytv and NBC: Stunt casting doesn't work all that well.

The show is at its funniest when it sticks to its core cast of crazies.

30 Rock is in its third season and on the surface, the array of high-profile guest-stars has been impressive.

Oprah Winfrey played herself -- uh, sort of. Turns out Liz Lemon (Fey) was hallucinating.

Jennifer Aniston played a wacky sexual predator who had trouble hanging onto a man.

Quite a stretch there.

Steve Martin played a white-collar criminal who tried to convince Liz to run away with him to Toronto, which is "just like New York, but without all the stuff." That's our favourite line of the TV season, hands-down.

Salma Hayek played a love-interest for Jack (Alec Baldwin) in a multi-episode arc.

And similarly, Mad Men's Jon Hamm played a love-interest for Liz in a multi-episode arc.

While some of those guest-stars contributed to the comedy in small doses, a couple of them merely were distracting. Actually, they were distracting to the point of being irritating.

The best guest star this season was Oprah, because she didn't completely take over the exquisitely written episode in which she appeared. A drugged-up Liz thought she was sitting next to Oprah on a plane, but it turned out it was just a "spunky little 'tween." Priceless.

There was some clever sending-up of Oprah's holier-than-thou image, too, with the other women of 30 Rock lining up to touch Liz, because she had touched Oprah (or so Liz thought).

The Martin episode was funny in places, too, but his story-line definitely took you out of the world of 30 Rock. And in a psychological sense if not a physical sense, we think that's always a mistake.

Aniston's character was way too frantic. She overwhelmed all of her scenes. It was almost as if 30 Rock's writers said to each other, "Oh my god, we have Jennifer Aniston for one episode, we have to get as much out of it as possible!"

Hamm was charming enough, as he always is. But again, his story-line had nothing to do with 30 Rock, per se. And that whole bit about him living in a "beautiful people bubble," while amusing in theory, went way over the top, don't you think? Those out-of-nowhere tennis scenes? Puh-leeze.

That said, the worst guest-star of the season on 30 Rock, by far, was Hayek. Talk about distracting. We never bought her character. We never bought her and Jack being together, Hayek's bodacious gorgeousness notwithstanding. And she seemed to stick around for-freakin'-ever.

Apparently Hayek initially agreed to do one episode, but was having so much fun with Baldwin in particular that she asked/agreed to do more. We're sure Baldwin wasn't complaining that he got to make out with Hayek for a few extra weeks.

We know stunt casting occasionally can help when a show is trying to build its audience, as 30 Rock is. But it must be used prudently. It doesn't do you any good long-term if your stunt-casting episodes also happen to be your weakest episodes.

Rest assured, we still love 30 Rock. But we want more of Liz and Jack. We want more of Tracy and Jenna. We want more of Frank the writer and Kenneth the page.

Put your foot down, Tina Fey.

As Jack says to Liz: "Be a manager."