Remember that the interview is a two-way street. Try to think about it from the point of view that you are also interviewing them to see if the job is suitable for you, and to see whether you want to work with them.
Too often people see it from the point of view of the interviewer having all the power - of thinking "will they want me for the job?"
But by asking "do I want to work for them?" - this can shift the way you perceive the balance of power in this situation, and that can help reduce the anxiety.