You should interview any agent prior to entering into an agent-client relationship. If more than one agent expresses interest in representing you, choose the one who seems most in sync with your vision of your work and writing future. Halfhearted representation is a setup for frustration and failure. A good agent will offer sound suggestions for getting your book in publishable shape. The agent needs to envision where your work belongs in the marketplace and have good contacts at appropriate publishing houses.
Agents handle submissions, negotiations, and contract revisions.
As the book goes through the publication process, the agent serves as your liaison to the publishing house and troubleshoots should problems arise. Generally, agents collect your advances and royalties. They deduct their fees and relevant expenses and send you a check for the remainder. You need someone capable of doing all of the above in a timely, professional fashion. If you are uncertain about an agent who agrees to represent you, ask for the names of clients you can contact for recommendations.