Art directors are the "creative" types within the business sector. It is their job to take the initial idea from the advertising department and make it sparkle. They provide the images to go with a copywriter's words and so the two must work in harmony to form the creative hub of a particular advertising campaign.
It is a very important role. The images produced here can be seen by thousands if not millions of people and the very best images produced by the director can even transcend the initial campaign and become iconic. The image of Santa with his rotund belly, jolly expression and red suit was popularized by a Coke-Cola campaign of the 1930s started by Haddon Sundblom. It's one of the most successful ad campaigns ever.
As the director breathes life into a copywriter's words, the two must be compatible. Art director's are known for their creative personality which is code for "temperamental" and must be able to communicate with the writer to make their given brief work.
Art Directors must not be too sensitive. Clients paying for campaigns naturally have some input and might ask for work to be tweaked. If they don't like a particular shade of blue, it is cut straight away and the art director must be able to take it on the chin and do has they are told.
Deadlines must be kept and presentation is often key. An idea must be "sold" to the client who will want to come away satisfied that their vision is being met even if it isn't and will therefore want to work with the company