40 Celebrities You Definitely Forgot Appeared on 'Friends'

In between shifts on ER, George Clooney and Noah Wyle appeared on Friends for two episodes. They served as dates for Rachel Green and Monica Geller, naturally.

The comedian guest-starred twice throughout the series — first as a stoned dinner guest in season 1 and later as a horrible blind date for Rachel in season 9.

The sitcom world collided when Mad About You star Helen Hunt appeared as Jamie Buchman in a scene at Central Perk. What most fans don't know, though, is that Lisa Kudrow used to have a reoccurring guest role on Helen's show as the ditzy waitress Ursula (who was then written into several storylines on Friends as Phoebe Buffay's identical twin sister).

Julia Roberts was one of Hollywood's biggest rising stars when she guest-starred as Susie Moss, a childhood friend of Chandler Bing who decides to ask him out in order to get even with him for embarrassing her when they were younger.

Following his debut film, Reality Bites, Steve Zahn appeared on season 2 as Phoebe's Canadian husband, whom she married so he could get his green card. "I still have people, and this was four days work in my life, around the world that come up and go 'Ah, Friends,'" Steve said on Entertainment Weekly's Couchsurfing.

The actress and model appeared on the episode "The One After the Superbowl" as a superfan stalker of the show Days of our Lives who has a hard time deciphering Joey Tribbiani from his character, Dr. Drake Ramoray.

Adam Goldberg was just beginning his career when he landed the reoccurring guest role of Eddie Menuek, Chandler's replacement roommate after Joey moves out. Adam went on to appear in films like Saving Private Ryan and A Beautiful Mind, as well as appear on TV shows like Entourage and Matt LeBlanc's Friends spinoff Joey.

Appearing as himself in season 2, Jean-Claude Van Damme's part involves him being recognized by Monica while filming a movie nearby, resulting in the two going on a date, as you might expect.

When Phoebe contracts the chicken pox right when her old flame Ryan is about to dock from the Navy, an adorably cute episode of Friends with Charlie Sheen ensues.

Throughout the Ross and Rachel saga, Christine Taylor made an interference as Ross's girlfriend Bonnie.

Coincidentally, Ben Stiller appeared on the show during the same season as his future wife, Christine Taylor. The actor and comedian appeared as Tommy, Rachel's date who has anger management issues.

The Italian actress and filmmaker Isabella Rossellini visited Central Perk as herself right after Ross decides she is "too international" to remain on his celebrity hall pass list. The hilarious cameo begins with him asking Isabella out, only to end with her finding out he bumped her off of his list for Winona Ryder.

It was by chance that Billy Crystal and Robin Williams happened to be on the same lot as Friends when the cast was filming "The One with the Ultimate Fighting Champion" in season 3. The comedians decided to stop by for a cup of coffee, during which an improvised conversation unfolds, leaving both the cast and audience in stitches.

Jon Favreau appeared throughout season 3 as a reoccurring guest star and Monica's love interest, Pete. The actor went on to produce and star in films such as Chef, Iron Man and The Avengers.

When Paget Brewster joined Friends for a multi-episode story arch as Joey (and Chandler's) girlfriend Kathy, she was fairly new in Hollywood. Since then, the actress has had stints on TV shows like Criminal Minds, Community, Law and Order: Special Victims Unit, and Modern Family.

Charlton Heston got to flex his signature tough guy act when he appeared as himself in season 4 of the series alongside Matt LeBlanc.

Tate Donovan dated Jennifer Aniston in real life throughout the first few seasons of the hit show, but when the actor appeared in season 4 as a reoccurring guest star (and Rachel Green's crush) the couple had already called off their engagement and parted ways. "It was horrible...I remember just getting back to my dressing room and just weeping," Tate told HuffPost Live.

Before his breakout role in Never Been Kissed, Michael Vartan guest-starred on the show as Dr. Tim Burke, the son of Monica's ex-boyfriend who she begins to date.

The show was a royal hit when the entire cast traveled to London for Ross and Emily's infamous nuptials — so much so that the Duchess of York, Sarah Ferguson, made a cameo in a scene with Matt LeBlanc.

When Rachel decides to fly across the pond for Ross's wedding, she shares the row with British actor Hugh Laurie, who is less than pleased to be seated next to her.