There's not much of an interview to speak of - the focus is on getting all the "employee" paperwork written up, and performing an "in-house" drug test (oral swab) to make sure you're not a "junkie" (a little less humiliating than providing a urine sample, I suppose). Usually, you won't be there interviewing unless the agency has a high probability of placing you on an assignment that is in the offing.

Very simple, lasted 20 minutes. Basic questions about job duties and what I know about the company. Received a call later in the morning and accepted job offer. Overall, a simple and pleasant process.