Writing scary scenes

I was writing a nightmare scene in my manuscript the other day and realized that I was scaring myself!  I saw the nightmare take place so clearly in my mind's eye that I had to get up and walk away from the computer for a little while. 

I've heard that Harriet Beecher Stowe cried for three days after writing the scene where Tom dies in Uncle Tom's Cabin and it makes me wonder if that kind of strong attachment to the story is similar for actors as well.