La La Land is a perfect movie, and Emma Stone and Ryan Gosling are perfect in it (want receipts? See: the Golden Globes). That said, the film could have been very, very different—in that director Damien Chazelle almost cast Emma Watson and Miles Teller as the leads.
"The casting of this movie during the six years it took to get made went through lots of permutations, and it's true there was a moment where Emma Watson and Miles Teller were doing it," Chazelle confirmed in a recent interview. "And neither of those casting things wound up lasting or working out. But it was part of the up and down of this movie: that we were about to make it, we were about to not make it, about to make it, about to not make it."
La La Land requires its leads to carry a pretty solid tune, and Emma Watson obviously would have done an *amazing* job (she sounds A+ in the trailers for Beauty and the Beast). That said, we can't imagine 2016's best film with anyone else than Stone and Gosling. Match made in movie heaven, etc, etc.