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.
Same. And if I’m struggling to cast a character, I’ll stop reading and google image search actors/celebrities with the described features and find someone to cast in that role (in my head) before moving on.
I find it difficult to engage with the character until I can put a face on them.
And if there is already a movie/show based on the book, I will sometimes just cheat and use that casting for visualization purposes.
I envy those of you who can do all this in your head.