Various artists may create or develop specific parts of an art piece or scene; but it is the charge of a sole art director to unify the vision. In particular, the art director is in charge of the overall visual appearance and how it communicates visually, stimulates moods, contrasts features, and psychologically appeals to a target audience. The art director makes decisions about visual elements used, what artistic style to use, and when to use motion.
One of the most difficult problems that art directors face is to translate desired moods, messages, concepts, and underdeveloped ideas into imagery. During the brainstorming process, art directors, coworkers, and clients are engaged in imagining what the finished piece or scene might look like. At times, an art director is ultimately responsible for solidifying the vision of the collective imagination while resolving conflicting agendas and inconsistencies between the various individual inputs. (source: http://bit.ly/PivCv )