Sometimes when I'm writing very fast, I forget to do this and have to backtrack. By "Live the Book," I mean that for every scene, I do my best to set the scene for myself. Be aware of everything in the room, how the room looks, what textures are there, what scents are present. What items other than furniture are there? If there's a couch, is it firm or soft, leather or cloth? How do I feel when I walk into that room? Details matter. They pull the reader into the story.
They also serve as handy alternatives to the common and sometimes repetitive tags. Instead of "What do you mean?" she asked, you can say Bev picked up the book and placed it back on the shelf. "What do you mean?"
In a world where pacing is king and dialogue and action make the story move, you don't want to overdo those kinds of tags or spend too much time on descriptions that bog down the story. Just because you know what the setting consists of (because you visualize it, smell it, touch it and hear it) doesn't mean that you have to put all the details on the page. You may not include more than one or two (or even none), but anything that helps the author live the book will ultimately lead to a richer story.