Justin Theroux was quite the furry-faced guy ... before he hooked up with Jennifer Aniston, that is.

In GQ's October issue, Jen's man poses as Frank Serpico, a character once played by Al Pacino in the 70's. As for why he has suddenly kept a clean-shaven face, Theroux tells the mag: "Certain chicks dig it; certain chicks don't."

Looks like this guy isn't taking any chances with Ms. Aniston. The Wanderlust actor adds, "You establish all sorts of mannerisms with it. Like when you're bored, you can feather your mustache."

With a new home in LA and romantic trips with his ladylove, doesn't look like he'll be bored anytime soon.