how do you deal with reading book first, then seeing movie, or the opposite?

Why I read it plays in my head with images, but not with definite character voices... I thought this was essential to read fast and folow an story and that every fast reader would have it.

I read all the LOTR books, with my own character faces/bodies in my internal movie.

then watched the movies.

The movies super-seeded my imagination, and then when I reread the books or read Silmarition or other Middle Earth stuff, it plays with movie faces/bodies. I kinda liked more the ones from myself, but I forgot them.