I do picture the action, the scene and the characters.
It is also easier if there is a movie related to the book, then picturing the characters is pretty sharp and easy as I just picture the actors and the locations from the movie.
On a rare occassion I become bogged down in a fast paced scene and I have to read and re-read sections so its clear where all the characters are and what they are doing, before I can move on.
If I am not really enjoying the book, then I move on pretty quickly and don't worry about placing every character and imagining the scene.
Sometimes for certain scenes I create a little side narrative that I feel should have happened in the book. (yea, yea, I know, seek professional help
)