It's important to listen to the interviewer, to pay attention, and to take time, if you need it, to compose an appropriate answer.
Also, be ready to engage the interviewer. You want there to be a give and take type of conversation, so you're building a relationship with the interviewer rather than just giving rote responses to questions. Have questions of your own ready to ask the interviewer.
Towards the end of the interview let the recruiter know that you believe the job is an excellent fit and that you are highly interested.