La La Land features plenty of singing and piano playing by the head actors. Were those real or were they dubbed over by other performers?
Its all him according to an interview:
"In fact, there's not a single close-up shot of [the character] Sebastian's hands in the entire movie that's a piano double," director Damien Chazelle said at a press event for the movie. "It's all Ryan." Even John Legend, who has a smaller part in the film, was impressed: "I was jealous, man," he said. "Watching him play, I was like, 'Wow, this guy is really good and he just learned this in the last few months.' It's pretty amazing."
According to Gosling, it took him three months of lessons to learn to play jazz piano: "The director wanted to shoot all those sequences in one take so there was no opportunity to squeeze in, or sneak in, a proper piano player." How is one person this talented?
Its all her too according to another interview
Were you nervous about singing in a movie?
I was nervous about it, but I also felt more prepared than I probably ever would have felt before, because I had been doing Cabaret eight times a week and I was able to go a little easier on myself about the fact that I am not, you know, the world's greatest singer by any stretch of the imagination and he really wanted it to be you know very realistic and natural and not technically perfect. So from the beginning he made it a less scary prospect if I need to go on screen and sound like Bette Midler, or something.:
While @sanpaco correctly mentions that the singing is real, he is incorrect about the piano sequences. In particular, according to Justin Hurwitz:
Hurwitz was wowed by Gosling’s commitment to learning the piano, specifically seeming to play the elaborate jazz solos laid down by veteran L.A. pianist Randy Kerber.
“There are no CGI hands, no piano doubles, he really had to learn it,” Hurwitz says.
So while the hands are real, the music was actually recorded separately by Randy Kerber.