Téa Leoni was the first choice for the role of Rachel Green before Jennifer Aniston was cast. However, Leoni turned down the offer due to scheduling conflicts with Bad Boys.
Eric McCormack was the runner-up for the role of Ross Geller before the casting of David Schwimmer — though not as much of a contender as anyone above, as the character was created with Schwimmer in mind.
Paul McCartney turned down the role of Emily's father. Yes, that Paul McCartney.
Matthew Perry and Jennifer Aniston were both signed on for different shows and had Friends in second position for the 1994-1995 TV season, meaning they could have become unavailable should the other shows have been picked up. Perry was cast in LAX 2194, a sci-fi sitcom about a futuristic LAX airport where aliens come and go daily. Luckily for Friends, studio executives saw just how awful that concept was and told the producers to go ahead and cast Perry as Chandler. Aniston was cast in Muddling Through, a CBS sitcom about a woman trying to rebuild her life after prison, with Aniston playing the main characters daughter. The Friends team liked Aniston so much that they worked out a scheduling agreement so she could shoot both shows, knowing that they'd have to reshoot multiple episodes if Muddling Through had any success. Aniston begged to be released from her Muddling Through contract and NBC set out to kill that seriesnote NBC started a scheduling war to tank Muddling Through's ratings and succeeded in just a few weeks so that she'd be available for Friends.
David Schwimmer had sworn off TV and film was going to focus on becoming a theater actor in Chicago when he was convinced to try out for Ross.
The opening title sequence was originally going to be based solely around the main cast playing around/goofing off in the fountain, but this version was only used once in the pilot episode and scrapped afterwards, due to the producers feeling that the scene showed the cast as a closed group of friends who might snub anyone outside their group. All of the later title sequences show a mixture of the fountain scene and scenes from other episodes.
R.E.M.'s 1991 song "Shiny Happy People" was the original choice for the theme tune of the show, which would've been a Real Song Theme Tune and in fact, the opening title sequence was originally different as it was made for the pilot, hence it was under the Working TitleFriends Like Us and included the song. After the show was picked up for a series as well as the band rejecting the offer to use "Shiny Happy People" (which they openly disliked) for the show, the producers wrote "I'll Be There for You" with the Rembrandts to replace it. Though the Rembrandts were always the producers' choice, R.E.M. was offered to perform the theme tune, but they declined it due to working on their album Monster at the time. They Might Be Giants was also offered but declined due to working on their album John Henry at the time.
Meanwhile Chandler was originally going to be gay, though they scrapped it when Matthew Perry was cast. Even afterward it's very obvious, especially in the early seasons, that they kept this idea on the table in case they decided to have Chandler come out. It is quite remarkable that Chandler and Monica would become such a stable couple. In any case, Chandler being Mistaken for Gay is a running gag throughout the show.
Joey and Monica were originally supposed to be the main couple focused on over the course of series rather than Ross and Rachel and later Chandler and Monica. A couple of episodes seem to have a few nods to this, such as Joey mistakenly thinking that Monica is flirting with him in "The One with the Flashback", and Monica revealing that she'd originally wanted to have sex with Joey right before getting together with Chandler in "The One With the Truth About London". There are also some subtle hints of this in "The Pilot": While it's unclear whether the writers changed the romantic focus to Ross and Rachel before or after the episode was filmed, the constant banter between Joey and Monica suggests that an eventual romance between the two was probably still on the cards when it was written.
Also connected with the Joey and Monica relationship, both their characters were originally written very differently. Joey was a self-centered Handsome Lech, and Monica was much more cynical and sexualized. However both Matt Leblanc and Courteney Cox took the roles in new directions which the writers loved (giving Joey 'heart' and making Monica the warmer, Team Mom of the gang) and they altered the script and dynamic. It was because of these changes that their relationship didn't really click anymore, and the writers quickly realized the much softened Monica was better paired with Chandler's insecurities and neuroses than with Joey's charm.
Although the Chandler/Monica relationship didn't come until Season 5, it almost happened much earlier. The writers first considered it during Season 2 and seriously pitched the idea at the beginning of Season 3, but nixed the plan as there was already a huge romantic plot with Ross and Rachel and they didn't want to split focus. In this version, Chandler's girlfriend would have been a crazy Trekkie who would leave him depressed by the end of the season, rather than Janice. After Ross and Rachel broke up they tested the waters for Monica and Chandler's relationship in the end of Season 3 and with the fans' enthusiastic response, began laying the groundwork throughout Season 4 and got them together for the finale.
They originally had the fallback of making Monica and Chandler a short fling and only lasting a few episodes into Season 5, which was why, in-universe, the relationship was kept a secret from the other characters. However the audience adored them so much and the characters fitted together so well that the writers continued with the relationship and they became the stable relationshipof the gang.
The original plan for the Season 5 finale involved Chandler cheating on Monica with a hotel employee while he was feeling depressed about Monica's recent lunch with Richard. Perry apparently rehearsed the scene with actress Lisa Cash, but when the time came to actually film it Perry refused, feeling that the fans would never forgive Chandler for cheating on Monica.
The Season 6 finale was originally going end with both Richard and Chandler proposing to Monica and leaving it as a cliffhanger of who she'd choose. This was scrapped because a) the producers weren't sure the show would be renewed for another season and didn't want to end it unconcluded and b) they realized it would be brain-searingly obvious that Monica would pick Chandler. The final result was a much more personal, romantic scene where Monica proposes to Chandler instead.
Also in the Season 6 finale, an additional cliffhanger planned for the episode was to have Ross discovering that Elizabeth had become pregnant, with the following season revealing that he wasn'tthe father. This was ultimately nixed due to the producers feeling it placed too much focus on a supporting character over the main cast and detracted from the main story about Chandler and Monica.
Lisa Kudrow and Matt LeBlanc came up with the idea that Phoebe and Joey had a Friends with Benefits relationship but this was immediately shot down by the producers. The two actors instead added a lot of flirting into their on-screen interactions to try and hint that it was going on.
They also considered having a more elaborate plot of Ross chasing after Rachel in the final episode, with him following her to Paris and them falling in love there but decided they wanted to keep things closer to home and the other characters.
Janine was supposed to stick around for more episodes but Elle Macpherson was based in London and wasn't available for more filming.
Originally Monica was the one having a baby in Season 8, but the writers thought it would open more storylines if Rachel did instead. To avoid having another pregnancy story in Season 10, they developed Chandler and Monica's infertility and eventual adoption arc.
The writers also believed that Season 8 would be the final season, so Rachel having her baby would've been the series finale. Once it was revealed that the show was renewed for another two seasons, a cliffhanger with Joey picking Ross's wedding ring up off the floor and gearing up to "propose" to Rachel was very clearly shoehorned in at the last second.
They also thought about revealing Monica was pregnant in the final episode but nixed it in favor of their adopted baby turning out to be twins. They envisioned they would have had a third child of their own anyway, much to the fans' happiness.
In-Universe, the appropriately titled "The One That Could Have Been" shows how things would end up if Monica never lost her excess weight, if Rachel went through with marrying Barry, if Phoebe had taken a high-stress job buying and selling stocks, if Ross and Carol had never gotten divorced, if Chandler had quit his job, and if Joey had remained on Days Of Our Lives.
Rachel's sister Jill was going to make more appearances, but Reese Witherspoon was too busy with her film commitments. Episodes intended to feature her were instead given to Rachel's other sister Amy, played by Christina Applegate, whose existence had only been vaguely implied before she debuted and who has a noticeably more obnoxious personality.
"The One Where Rachel Tells Ross" was originally supposed to have Chandler and Monica not arriving at their honeymoon. Chandler makes a joke about bombs at the airport and the two are detained. However, it was changed after the September 11 attacks (though the scenes did surface online over a decade after the episode aired and were later officially released on the complete series Blu-Ray set).
There was originally going to be a spin-off called Girlfriends, where Phoebe and Charlie move in together.
In addition to Girlfriends, there were also plans for a spin-off focusing on Rachel as well as a spin-off focusing on Monica and Chandler but neither actor were interested, so instead we got Joey.
Patsy Kensit was the original choice to play Emily but she turned it down.
Not only that but Ross' marriage with Emily was going to last longer and would've likely ended in the season five finale than midway into the season. However, David Schwimmer was unhappy with his character being remarried and Helen Baxendale was pregnant with her first child, which prompted her return back to the United Kingdom, thus this led to said plans to be nixed.
Emily was also planned to return in the final season, possibly for a Love Triangle storyline between her, Ross and Rachel (similar to the one between Phoebe, Mike and David) but Baxendale declined the offer as she was busy with her commitments on the British West End play After Miss Julie and didn't want any tabloid attention like she did back in 1998.
There were plans for Rachel and Gunther to become a couple but neither Jennifer Aniston nor James Michael Tyler were interested.
In a May 2021 interview promoting the reunion special, Kevin S. Bright revealed that there were plans to do an episode set in Disney World in which Joey would've gotten a summer job there and would go on a date with an actress who was playing Cinderella in the parade.
Writer Peter Mehlman revealed that NBC suggested a crossover between Friends and Seinfeld. The event would take place in two episodes aired on the same night, with the characters from Seinfeld appearing on Friends, and vice versa. Larry David immediately rejected the idea (although Jason Alexander later had a one-off cameo as a suicidal office worker Phoebe convinces to keep on living). This would have lead to a You Look Familiar situation, as Courteney Cox played Meryl, a girlfriend of Jerry's, in an episode of Seinfeld six months before Friends aired.
Chandler's father was planned to be a recurring guest star throughout the series, the idea was that he'd be a celebrity drag impersonator and so they'd use him visiting as an excuse to cast different female celebrity guest stars. Ultimately they never used the idea and he only appeared twice, both times portrayed by the same actress.
Alice was planned to have a larger role in “The One Hundredth”, but Debra Jo Rupp was starring on That '70s Show by then and was only available for one scene.
as it was made for the pilot, hence it was under the Working Title Friends Like Us and included the song. After the show was picked up for a series as well as the band rejecting the offer to use "Shiny Happy People" (which they openly disliked) for the show, the producers wrote "I'll Be There for You" with the Rembrandts to replace it. Though the Rembrandts were always the producers' choice, R.E.M. was offered to perform the theme tune, but they declined it due to working on their album Monster at the time. They Might Be Giants was also offered but declined due to working on their album John Henry at the time.