As far as I know, “watch out” and “look out” are interchangeable expressions for warning someone in the face of an imminent danger to that person’s safety or well-being. I would think, though, that “Look out!”—with the exclamation mark—has greater immediacy than “Watch out!” “Look out!” is more likely to be uttered by someone actually seeing a person at the very precise moment of getting in harm’s way, like being in the path of a speeding car or a falling piece of roofing. On the other hand, “Watch out!” can apply not only to such immediate situations but also to anticipated ones, as in making a parting word to someone about the risk of meeting a troublesome person sometime later; in this latter case, it’s more likely for people to say “Well, see him if you must, but watch out!” than “Well, see him if you must, but look out!”
