Billy Crystal and Robin Williams, two titans of comedy, made a memorable appearance as Tomas and Tim, two friends who hilariously interrupt Monica's love life update with their own relationship woes. Their comedic timing and chemistry added a unique flavor to the episode, leaving viewers in stitches. Their characters' unexpected intrusion at Central Perk and their absurdly funny conversation about their personal lives showcased the comedic prowess of these two legendary actors, making their guest appearance a standout in the series.

At the height of their "ER" fame, George Clooney and Noah Wyle guest-starred as two doctors who tended to Rachel's injured ankle, leading to a disastrous double date with Rachel and Monica. Their characters, Dr. Mitchell and Dr. Rosen, were a nod to their roles on "ER," and their appearance added a touch of medical drama to the sitcom. The double date, filled with mistaken identities and awkward moments, was a comedic highlight of the episode, showcasing the charm and wit of these two renowned actors.

Reese Witherspoon played Rachel's shopping-obsessed younger sister, Jill, who stirs up drama when she starts dating Ross. Witherspoon's portrayal of the spoiled and self-centered Jill was spot-on, providing plenty of laughs and adding an extra layer of complexity to the Green family dynamics. Her attempts to mimic Rachel's life, including pursuing Ross, created a delightful tension that made her episodes some of the most memorable. Witherspoon's comedic timing and chemistry with the cast shone through in this role.

Julia Roberts guest-starred as Susie Moss, a grade school friend of Chandler's, who he embarrassed in front of the whole school in the fourth grade. Their reunion leads to an unexpected twist as Susie seeks revenge on Chandler for his childhood prank. Roberts' charismatic performance and the surprising plot twist made this episode a fan favorite. Her character's playful yet vengeful nature added a unique dynamic to the show, and her romantic scenes with Chandler were filled with humor and charm.

Bruce Willis played Paul Stevens, the father of Ross's girlfriend, who uses his imposing build to intimidate Ross. Willis won an Emmy for this role, where he showcased his ability to play a character that was intimidating yet vulnerable. His character's protective nature over his daughter and his subsequent relationship with Rachel added a new dimension to the series. Willis' performance was both funny and touching, proving that he could hold his own in the world of sitcoms.

Brad Pitt played Will Colbert, a former high school classmate who was bullied by Rachel and founded the "I Hate Rachel Greene" Club with Ross. Pitt's guest appearance was notable not just for his performance but also for his marriage to Jennifer Aniston, who played Rachel, at the time. His character's grudge against Rachel and his unexpected friendship with Ross added an interesting twist to the Thanksgiving episode, making it one of the most memorable in the series.

Despite their significant age difference, Tom Selleck played Dr. Richard Burke, a love interest for Monica. Selleck's charismatic performance and his character's mature and charming personality made him a fan favorite. His relationship with Monica was a crucial storyline in the series, and their breakup was one of the show's most emotional moments. Despite the age difference, their chemistry was undeniable, and Selleck's portrayal of Richard as a kind, loving, and ultimately heartbroken character added depth to the series.

Gabrielle Union appeared as Kristen, a new neighbor who unwittingly becomes the object of affection for both Ross and Joey. Union's character was caught in a hilarious love triangle, with both Ross and Joey trying to win her over. Her confusion and amusement over their antics added a fun dynamic to the show. Union's performance was charming and humorous, and her character's interactions with Ross and Joey provided some of the series' most comedic moments.

Hugh Laurie played the man Rachel sits next to on her flight to London, who is completely unsympathetic to Rachel's side of the story. His character's blunt and sarcastic remarks to Rachel's dramatic revelations made for some of the most hilarious moments in the series. Laurie's dry wit and deadpan delivery were a perfect fit for the role, and his character's indifference to Rachel's dilemma added a unique comedic element to the episode.

Charlton Heston played himself in an episode where Joey embarrasses himself by using Heston's dressing room shower. Heston's cameo was a humorous nod to his status as a Hollywood legend, and his reaction to Joey's awkward situation was priceless. His stern yet understanding response to Joey's apology added a touch of warmth to the episode, showcasing Heston's ability to bring depth and humanity to even a brief cameo role.

Denise Richards played Ross and Monica's cousin, Cassie, whose beauty leaves both Chandler and Ross stunned. Richards' portrayal of the attractive and oblivious Cassie was both funny and endearing. Her character's unawareness of the effect she has on men, particularly Ross and Chandler, led to some hilariously awkward moments. Richards' performance added a touch of glamour and comedy to the series, making her guest appearance a memorable one.

Paul Rudd played Mike Hannigan, Phoebe's third husband, who she meets by chance when Joey forgets to set her up on a blind date. Rudd's character was a perfect match for Phoebe's quirky and eccentric personality. His acceptance and love for Phoebe, despite her unconventional ways, made their relationship one of the most endearing in the series. Rudd's comedic timing and chemistry with Lisa Kudrow made his character a fan favorite.

Before his fame from "The Suite Life of Zack & Cody," Cole Sprouse played Ross's son, Ben, who delights in pranks. Sprouse's portrayal of the mischievous Ben was both adorable and hilarious. His character's pranks, particularly the one where he tricks Ross into believing he can't say "Daddy," showcased Sprouse's comedic talent at a young age. His interactions with Ross and the rest of the cast added a touch of youthful humor to the series.

Isabella Rossellini played herself in an episode where Ross regrets bumping her off his list of celebrities he's allowed to sleep with. Rossellini's cameo was a humorous nod to her status as an international icon. Her reaction to Ross's list and her subsequent dismissal of him were some of the funniest moments in the series. Rossellini's cameo added a touch of glamour and humor to the show, making her guest appearance a memorable one.

Freddie Prinze Jr. played Sandy, a male nanny whose sensitivity makes Ross uncomfortable. Prinze Jr.'s portrayal of the caring and emotional Sandy was both humorous and endearing. His character's unconventional approach to childcare, including puppetry and homemade cookies, was a source of amusement for the other characters and the audience. Despite Ross's initial discomfort, Sandy's kindness and dedication to his job made him a memorable character in the series.

Jon Favreau played Monica's boyfriend, Pete Becker, a wealthy corporate computer manager who becomes obsessed with becoming a UFC fighter. Favreau's character was a unique addition to the series, with his ambitious dream adding an unexpected twist to his relationship with Monica. Despite his lack of fighting skills, Pete's determination and optimism were endearing, and Favreau's performance added a touch of humor and charm to the series.

Anna Faris played the birth mother of Chandler and Monica's twins, who initially isn't sure who the father of her baby is. Faris's portrayal of the naive and sweet-natured Erica was both funny and touching. Her character's interactions with Chandler and Monica, particularly her confusion over Chandler's job, provided plenty of laughs. Faris's performance added a touch of sweetness and humor to the series, making her guest appearance a memorable one.

Dakota Fanning played the girl moving out of the house Chandler and Monica intended to buy, bonding with Joey over their shared desire for the couple not to move. Fanning's character, Mackenzie, was a wise-beyond-her-years child who provided a unique perspective on the situation. Her heartfelt conversation with Joey about change was a touching moment in the series. Fanning's performance added a touch of innocence and wisdom to the show, making her guest appearance a standout.

Before starring on "Criminal Minds," Paget Brewster played Kathy, who first dates Joey but later falls for Chandler, leading to a complicated love triangle. Brewster's character was a source of tension and drama in the series, with her relationship with Chandler testing his friendship with Joey. Despite the complications, Kathy's character added depth to the series, and Brewster's performance was both charming and compelling.

Aisha Tyler played a paleontology professor who initially falls for Joey's confidence but later starts a relationship with Ross. Tyler's character, Charlie, was a unique addition to the series, with her intellectual pursuits contrasting with the other characters. Her relationship with Ross, and their shared interest in paleontology, added a new dynamic to the series. Tyler's performance was both funny and engaging, making her character a memorable part of the series.

Susan Sarandon played Cecilia Monroe, a "Days of Our Lives" actress whose character's brain is transplanted into Joey's character. Sarandon's character was a humorous nod to the world of soap operas, with her dramatic performance providing plenty of laughs. Her character's interactions with Joey, particularly her advice on how to portray her character, were some of the funniest moments in the series. Sarandon's performance added a touch of drama and humor to the show, making her guest appearance a standout.

Christina Applegate played Rachel's other sister, Amy, who is bossy, abrasive, and causes drama around baby Emma. Applegate's portrayal of the self-centered and clueless Amy was both hilarious and infuriating. Her character's disregard for others and her outrageous comments provided plenty of laughs. Despite her character's flaws, Applegate's performance was both funny and engaging, making her guest appearance a memorable one.

Charlie Sheen played U.S. Navy Lieutenant Commander Ryan, or as Phoebe calls him, "my 'submarine guy.'" Sheen's character was a unique love interest for Phoebe, with his military background and limited time on land adding an interesting dynamic to their relationship. Despite their brief romance, Ryan's character left a lasting impression, and Sheen's performance was both charming and humorous.

Sean Penn played the ex-fiancé of Phoebe's twin sister, Ursula, who falls for Phoebe after meeting her. Penn's character, Eric, was a kind and naive man who was unknowingly engaged to the manipulative Ursula. His relationship with Phoebe, and his discovery of Ursula's deceit, added a touch of drama and romance to the series. Penn's performance was both touching and funny, making his guest appearance a memorable one.