Normally in the illustration field, there is a hierarchy of people you speak to while you are working; most of the time this hierarchy will change depending on how you decide to go about working through the industry.
As an illustrator you can expect to work with any one from another illustrator, to a graphic designer, to the client, even an art director. The hierarchy is such that can be arranged neatly into a series of flow charts.
Client – Agent – Illustrator – Graphic Designer
In the above instance, the agent could possibly work with the graphic designer as well as the illustrator.
Agency – Illustrator – Graphic Designer
Here the graphic designer could be working alongside the illustrator.
Art Director – Illustrator
When the art director directly approaches the artist for work.
Agency – Art Director – Illustrator – Graphic Designer
In this instance, the art director could contact both the graphic designer or illustrator, and get the two to work together to create a piece, or only contact one.
Client – Art Director – Illustrator – Graphic Designer
The client in this situation could be the leader of a company deciding they want to get some artwork done for something within said company and so they would talk to the designated Art Director who would go and hire the artist who would go on to complete the work.