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.
I’m not able to visualize things in my head (aphantasia), so I don’t! I can imagine how frustrating it would be to picture something and have that challenged; the closest I can imagine is when I finally see, on video, a podcaster I listen to regularly. It is so jarring because it’s as if different parts of the brain are fighting to control the concept of the person.
My mom has always told me that she casts the movie in her mind when she reads a book that really grabs her.