The roles are juicy, Emmys are possible and an audience of millions is inevitable. Big pond, big splash.

Guest roles on top TV shows have become the desired destination for A-list actors, fading stars, fashion gurus and even politicians. And why not? These little gems require less time and grind than a starring role but provide big career boosts -- think of Brooke Shields and Christina Applegate jump-starting their careers on "Friends."

This past Thursday, ABC's "Ugly Betty" and "Grey's Anatomy" returned with new episodes, and the stars are lined up in galaxies.

This Thursday, Christian Siriano, the impish designer who won "Project Runway," joins a long list of guest stars who have made a splash on the stylish "Betty." In the same episode, Rita Moreno will play a member of the Suarez family, and appearances will be made by actors Eddie Cibrian and Rebecca Gayheart and by "Project Runway" judge Nina Garcia.

From the beginning, "Betty" has been awash in guest actors (Lucy Liu, Patti LuPone, Kristin Chenoweth and Victor Garber, to name a few) and celebrities from fashion (Kenneth Cole, Vera Wang, Isaac Mizrahi) and music (Victoria Beckham, "American Idol" finalist Katharine McPhee and judge Simon Cowell).

The guest spots on "Betty" tend to be quick and breezy. The spots are a pretty showcase seen by about 20 million people, many of whom are the much-coveted young, well-educated and well-to-do demographic.

The guests who grace "Grey's Anatomy," however, tend to be slightly more serious and usually make more than one appearance in substantial story arcs. Amy Madigan has signed on for five episodes as a psychiatrist. No details on the role, but Cheech Marin also was in this past Thursday's episode.

Earlier in the season on "Grey's," we saw Seth Green spurt blood from his neck and croak, and Edward Herrmann, who won an Emmy for a guest shot on "The Practice," play a bumbling intern.

Not all guest stars need a career boost, but TV can give them one anyway. Blockbuster movie star Julia Roberts took a minor role on "Law & Order" a few years ago, when she was dating then-cast member Benjamin Bratt. And lest we forget, Brad Pitt popped up on "Friends" when he was married to Jennifer Aniston.

Casting against type is a big deal in guest spots. Child actor Fred Savage ("The Wonder Years") went from young and sweet to adult and sinister in a chilling guest shot on a 2003 episode of "Law & Order: Special Victims Unit." And nobody who saw Henry Winkler as that super-creepy bad guy on "SVU" will ever again think of Fonzie on "Happy Days."

More than a few "seniors" have been introduced or re-introduced to the TV audience in guest shots. Sometimes they play parents; sometimes they play judges, lawyers, criminals and even love interests.

Polly Bergen and Dixie Carter both played parental units on "Desperate Housewives." Debbie Reynolds and Blythe Danner were moms on "Will & Grace," and Elliott Gould, Morgan Fairchild and Marlo Thomas played parents on "Friends."

In the nonfamilial vein, Tom Selleck took a wonderful turn as Shirley's former lover in a multi-episode arc on "Boston Legal" (he also played Monica's older lover on "Friends"), and Michael J. Fox, who has Parkinson's disease in real life, played a millionaire dying of cancer and courting an attorney on "Legal" as well.

Also on "Boston Legal," veteran stars Shelley Berman and Henry Gibson have played judges, Delta Burke took a hilarious turn as a paramour of Denny Crane, and Betty White played Alan Shore's sweet but murderous secretary.

At least two shows have become super-famous for their plethora of top guests. Every week there's somebody at least recognizable and often big-time on "Law & Order" and "ER." Dick Wolf, creator of the entire "L&O" franchise, boasts that if a New York actor has a SAG card, he or she has been on his shows at least once.

The guest list for "ER" goes on for miles, perhaps most famously with Sally Field portraying Abby's manic-depressive mother (and winning an Emmy for it and landing her own series, "Brothers & Sisters"), Bob Newhart as a lonely terminal patient (Emmy-nominated) and Forest Whitaker as a homicidal ex-patient (also Emmy-nominated).

Many of these quick hits are far more memorable than the main roles -- and often more memorable than the guest star's own starring roles. It's a spark for them and a spark for us. Keep 'em coming.