One blob of red in the wrong place and the audience isn't looking at the hero, they're looking at a patch of curtain (or something similar) and your whole effect is lost.
“All the time he's boxing, he's thinking. All the time he was thinking, I was hitting him.”
“As soon as anybody puts anything on film, it automatically has a point of view, and it's somebody else's point of view, and it's impossible for it to be yours.”