Essentially, the Art Director defines the artist style and helps develop it among the team. To me it seems like it is one of the most important roles within the art department, because if the overall style is off and not exciting to look at, people aren't going to want to look at it (putting it simply).
Art Direction is evident in most forms of media, but film is probably the area in which a art director would get the most 'kudos'. I do not think that games and films are so different, other than the fact that in a games the player has choice on what he/she sees and how events unfold, although cut scenes and certain games would be quite similar. In fact, games could prove more of a challenge since what is seen by the player is not predefined.
Proof that they are not dissimilar is shown by Jerry O'Flaherty, previously the Art Director for Gears of War and Unreal Tournament 3, being made the director for the upcoming CG-animated Thundercats film.
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So to be an Art Director would require a massive understand of all things art. Lighting, color, perspective, scale, and composition. A big understanding in all of the jobs would be necesscary, such as, modeling, concept art and the game engine.
But all of these would mean nothing without a creative imagination and a passion for games.