Hi Nina,
Art director/creative director are the people who control the look and feel of the job.
When the account executive of an advertising agency wins a print job after tendering, he would brief the creative/art director of the agency. Art director would meet with the client with or without his team of graphic designers and copywriter (depending on size of job). After thorough discussion with client, he would produce visuals with his team to present to client for approval and to sign off. The visuals could contain page layouts, images done with colour magic markers or line sketched, etc. When they have been signed off (approved), then the photographer or illustrator would be commissioned to produce these images. The photographer or illustrator has to follow the visual precisely. Therefore, the creativity had been decided even before the photographer is commissioned.
Generally, there are two types of photographic work, one is commissioned with strict instructions. The other is either speculative or commissioned without instructions (normally by inexperienced or uncommitted clients) where you have a free hand and be as creative as you see fit.
So answer to Robin's question, if an art director is involved, there is no need to worry about the composition, because he is the one who decides on composition, background colour, props, camera angle, mood of the shots, etc.....