"(Director) David Fincher walked up right before we did the first one and just said, "Hit him in the ear". I said, "Really? I'm not sure, we're just getting going on this, he doesn't know me so well..." but I went and just kinda clipped him with the point of my thumb and it's in the scene."
"He says, Why the ear?" he added.
The line was added to the movie even though it did not exist in the script, as Fincher liked it very much.