It's hard to imagine anyone else playing Rachel Green in Friends, but apparently, the role could have gone to Saved by the Bell's Tiffani Thiessen, who also auditioned to play Rachel. During an October 2018 appearance on Nikki Glaser's SiriusXM show, Thiessen revealed that she did a screen test but ultimately wasn't the right fit, as she was only 20 at the time. "I was just a little too young," she explained. "I was a little too young to the pairing of the rest of them."
Even after producers decided that they wanted Jennifer Aniston, scheduling conflicts kept her from committing. Before getting cast on Friends, Aniston landed a role in the CBS comedy Muddling Through, and according to her contract, she wasn't allowed to be on any other shows unless Muddling Through was cancelled. "The fear was that we would shoot four or five [episodes], and CBS, just to screw with NBC, would pick up Muddling Through, and then we would have to recast and reshoot the first five episodes," co-creator David Crane explained to The Wrap in June 2018.
According to Saul Austerlitz's book, Generation Friends, NBC president Warren Littlefield purposefully aired TV movies adapted from Danielle Steel novels in the same time slot as Muddling Through in order to ensure its cancellation. The plan worked: the CBS sitcom was cancelled due to low ratings, and Aniston was able to commit to Friends.