I like to act out my own scenes sometimes. (When no one is home; I definitely cannot act). I like to do this, though, because it gives me an idea of what exactly is going on in the scene. It gives me a really clear picture of what I want to happen, because usually I can only describe what's happening if I can see it first.
The problem with this is I see everything, so my first instinct is to write everything. I think, "well, I see him turn on his back and do this, so I must describe every little movement so the reader knows what's happening."
Every book would be three times as long if every writer did that. The thing is, the reader doesn't need to know what I see in my head. I give them enough words so that they can take the words and use their imagination to fill in the rest. It's amazing what you can do with a few words. It's easy, for me anyway, to think if I describe it enough, they'll see it perfectly. In fact, I think the opposite is true. You just have use the exact perfect words so that those words trigger the imagination in the way you want them to, so they see a bit of the picture you see (of course, it will never be the same picture).
And that's really hard. But whoever said writing was easy?