I saw "La La Land" a couple of times this week. Once with a friend and once by myself. I actually got more out of it the second time and for some reason found myself particularly focused on Emma Stone. The good actors, and Emma is certainly one of the best of her generation, are capable of a wide range of facial expressions, voices, body language and other devices to make you believe in the characters they play.
I have no idea how many hours it took her to learn her part for "La La Land," but my hunch is it was a lot. Her performance (like a Michelle Williams or Meryl Streep) while seemingly off the cuff at times, struck me as well rehearsed and polished. An actor who knows what effect she's after and how to make it happen on film.