Personally I give each character a face, either from a person I know who matches their personality or a celebrity that fits the description, even though sometimes it can be really annoying if the author gives more details on the persons appearance and my character doesn’t fit that lol. I’m very curious as to what other people do.
Mostly by the cover. Or some other semi-official pre-made art (like I think the Hobbit characters looking like how they pictured in certain comic book). . Some authors describe their characters in clear detail so I just know how they look. Sometimes I compare them to similar characters and think that they look quite the same.