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Reese Witherspoon
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Reese Witherspoon appeared in a couple episodes of "Friends" as Rachel's spoiled sister Jill. She even goes on a date with Ross at one point, just to get under Rachel's skin. Clearly it didn't have an ill-affect on Rachel and Jill's relationship as Witherspoon and Jennifer Aniston are set to share the screen again, co-starring in Apple's upcoming series "The Morning Show" which will take viewers behind-the-scenes of a "Today"-like morning infotainment show.
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Christina Applegate
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We all know Rachel has two sisters. While Reese Witherspoon played spoiled sister Jill, Christina Applegate played her other sister, the equally-spoiled and self-involved Amy. Applegate's guest appearance earned her a Guest Actress in a Comedy Series Emmy in 2003.
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Julia Roberts
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Julia Roberts was already a mega star in 1996 when she appeared in a two-part post-Super Bowl episode of "Friends" playing Susie Moss, a woman with whom Chandler went to elementary school. "Susie Underpants" as she was nicknamed after a childhood prank pulled on her by Chandler, exacts her revenge by going on a date with Chandler and leaving him without pants in a public washroom.
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Brooke Shields
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In the same post-Super Bowl episode that featured Julia Roberts, Brooke Shields guest-starred as Erika, a "Days of our Lives" fan obsessed with Joey or rather, his character Dr. Drake Ramoray. Erika freaks out the friends group when she clearly has trouble differentiating fact from fiction. The episode wasn't the last time Brooke Shields would appear on an NBC sitcom - she starred in her own show, "Suddenly Susan" which premiered in September of that same year.
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George Clooney and Noah Wyle
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For many years, both "Friends" and the medical drama "ER" were part of the NBC "Must See TV" super successful Thursday night lineup which, every once in a while, featured crossovers with show casts. In 1995 George Clooney and Noah Wyle, who played handsome doctors Doug Ross and John Kyle on "ER" showed up in an episode of "Friends" playing, wait for it, handsome doctors who go on a double date with Monica and Rachel. Strangely enough, Clooney and Wyle didn't play their "ER" characters, instead playing doctors Mitchell and Rosen.
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Bruce Willis
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Bruce Willis is known for his many tough guy roles on the big screen but on "Friends" he guest-starred as Paul, the father of Ross's young girlfriend Elizabeth who intimidates Ross, but lets down his hyper-masculine persona while dating Rachel.
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Winona Ryder
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Before her comeback in the Netflix series "Stranger Things" Winona Ryder appeared in an episode in the seventh season of "Friends" playing Melissa, a former sorority sister of Rachel's. Over the course of the episode it's revealed Melissa and Rachel shared a steamy kiss in college which Phoebe finds hard to believe and Joey tries extra hard to believe.
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Hugh Laurie
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Years before he starred on his own medical drama, "House" Hugh Laurie appeared in the 1998 season four two-part finale of "Friends" called 'The One With Ross's Wedding' where Rachel makes the last-minute decision to fly to London to tell Ross she still loves him before he marries Emily. Laurie plays a British man seated next to Rachel on her flight who gets annoyed with her, calls her a "horrible horrible person," discourages her from telling Ross how she feels and just when you think he can't serve her anymore tea, he hits her with, "it seems to be perfectly clear that you were on a break" referring to the the infamous and contentious "we were on a break" running storyline.
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Ben Stiller
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Though the relationship between Ross and Rachel was one of the main storylines throughout the run of "Friends," both characters did their fair share of dating other people. In an episode in the show's third season, Ben Stiller played one of Rachel's dates who has major anger issues although, for the majority of the episode, it's only Ross who can see it.
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Morgan Fairchild
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Morgan Fairchild was a staple of 80s and 90s soaps and appeared on "Friends" in several episodes as Chandler's romance novelist mother, Nora. She makes Chandler uncomfortable with her forthright sex talk and the fact that she kissed Ross in one episode.
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Charlie Sheen
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Long before he was promoting the virtues of tiger blood, Charlie Sheen guest-starred on an episode of the second season of "Friends" playing Ryan, a naval officer and one of Phoebe's former lovers who is in town for what she hopes will be a hot and heavy reunion. Unfortunately his visit happens at the same time Phoebe is dealing with a case of chicken pox and they both spend their time itching and scratching.
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Robin Williams and Billy Crystal
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Comedy legends Robin Williams and Billy Crystal turned up in the third season as two men who slide onto the couch beside the gang at Central Perk and proceed to have a wildly personal (and loud) conversation which the friends can't help but eavesdrop. The appearance was a stunt to promote the pair's 1997 comedy, "Fathers' Day" which hit theatres the day after the "Friends" episode aired.
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Danny DeVito
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In the final season of the series, Phoebe gets engaged to boyfriend Mike (played by ageless wonder Paul Rudd) which sees Monica and Rachel throw her a bachelorette party complete with an um...unconventional stripper played by Danny DeVito. Considering the episode is called 'The One Where The Stripper Cries' it's safe to say the party doesn't exactly goes as planned.
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Susan Sarandon
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Oscar-winner Susan Sarandon showed up in the seventh season of the series, playing Jessica Lockhart who costars with Joey on Days of our Lives. Things get typically soap opera dramatic when it's revealed Jessica will be killed off and her brain donated to Joey's character who is in a coma. Jessica does not take well to the news.
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Brad Pitt
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Over the course of its ten seasons, the Thanksgiving episodes of "Friends" became some of the show's most iconic including one in the eighth season which featured Brad Pitt as Will, a former high school classmate of Monica, Rachel and Ross. Monica runs into the previously-overweight Will and invites him to Thanksgiving dinner where it's revealed he not only hated Rachel in high school (and still does) he, along with Ross, formed an "I Hate Rachel" club. The episode was made even more hilarious because Brad Pitt and Jennifer Aniston were married at the time.