In reading a fictional novel, how do you visualize the story and characters?

I was reading a newly bought book called “The Ridge” and quickly imagined the main character as Harrison Ford. But getting to the next chapter, he becomes George Clooney in my head. I’m afraid in the next few pages, he could turn into me since a sexual encounter might be forthcoming. : )

So how do you visualize the scenes and characters while reading a novel? Do you assign certain original images and faces and stick with that or do they keep changing as you progress?