More from EW
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''The One With the Rumor'' (Season 8)
Jennifer Aniston's then husband starred in a Thanksgiving episode, playing a very different role: a founder member of the ''I Hate Rachel'' club.
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''The One With Rachel's Sister'' (Season 6)
The actress was utterly perfect as Rachel's even-more-spoiled little sister Jill, who shook up her sib when she briefly dated Ross.
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Pheobe's best chance at normalcy was marrying geeky pianophile Mike, who became a beloved member of the group during 18 episodes.
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''The One With Rachel's Other Sister'' (Season 9)
Rachel's other sister was the despicably witless Amy, who didn't know her niece's name yet still wanted custody if her parents died.
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''The One With Joey's New Brain'' (Season 7)
Sarandon played a soap diva (think Susan Lucci) who struck up a romance with Joey after he let it slip her character would be killed.
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''The One With Rachel's Big Kiss'' (Season 7)
Nobody believed that Rachel had actually kissed her former sorority sister, played with hilarious closeted enthusiasm by Ryder.
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''The One Where Paul's the Man'' (Season 6)
The action star won an Emmy—and got to hook up with Jennifer Aniston—playing the intimidating father of Ross' student girlfriend.
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Seasons 2,3, and 6
Magnum P.I. himself romanced Monica as Dr. Richard Burke, a friend of Monica's father and Chandler's biggest competition.
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''The One After the Super Bowl: Part 2'' (Season 2)
Roberts got sweet revenge as Susie, who stole Chandler's clothes, stranding him in the restroom.
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George Clooney and Noah Wyle
''The One With Two Parts'' (Season 1)
The ER hunks appeared as—this is a stretch—cute doctors who went out with Monica and Rachel.