I interview for admin type roles quite a lot.
Be pleasant as make a bit of small talk as you walk to the interview etc as well as the person being able to do the role I want to see a person who will fit in with my team. I often have people who I can't really decide between scoring wise - I then go on personality and fit. If you were pleasant and friendly I'm much more likely to offer it to you than the miserable one.
Don't feel you have to blurt out an answer as soon as it's asked. Take a sip of water. Pause. It's fine. It's not a speed competition. I'm also quite happy for people to have a pen and paper and jot things down before they answer.
Research the company. I interview so many people who don't seem to understand the role they've come for at all.
Don't ask about terms and conditions in the final questions bit. So many people do this. It puts me off if the first thing they jump in with is 'what hours can I work' etc - I'm really flexible but I don't want this to be the only thing people are thinking of - if you get offered the role then go ahead and make sure it will fit with your life before you hand your notice in elsewhere.
It's useful for you to ask as your final question (after something about the job) what is going to happen next. When will you expect to hear etc as so many interviewers neglect to tell candidates this and then you're left in limbo.
Thank the interviewers for their time at the end.
Remember - you are interviewing them as much as you are being interviewed - think 'they would be lucky to have me' and if you would want to work with them - I find this helps people with nerves.