Interview day is like your wedding day - you need to be sure you want to do it, and if you aren't, then right up to the threshold it's not too late to withdraw. But it's more difficult to go back once the knot is tied.
Don't, though, confuse genuine doubts with nervousness. The most self-assured person in the waiting room is just putting on a bold front.
Remember that and you're well on the way to handling your own fear.
Try to rehearse your interview if you can. Some professional development courses lay this on, and if not, there are colleagues who are ready to help - but pick someone who has genuine experience of being on the other side of the table. Even then, treat what's said as advice rather than instruction, because only you really understand what makes you feel confident and comfortable.
Part of the rehearsal should cover basic things such as how you walk into the room and sit down. I remember an interview where the head showed me into the room, and then stood back and gestured towards the panel (memory puts them 200 feet away, but that's probably wrong) as if to say, "Here he is. Take a good look". So don't go all Uriah Heep and nervous. Look confident, smile, and walk to your chair. If you're not sure where to sit, and the panel are too ill-mannered to tell you, then say, "Shall I sit here?"
They may - and again, it's a bit ill-mannered - still be writing and muttering about the previous applicant. If so, sit still. Don't fidget.
Keep a straight face. Sit up, in repose but not rigid. Keep your hands still - folded in the lap is good. Don't scratch, swap bum cheeks or emit nervous coughs and yelps.
Someone on the panel will check basics - your name, present job. If they get it wrong, tell them, but do it politely because they don't appreciate being wrong-footed at the start.
There will be a set of pre-arranged questions, asked by panel members in turn. They may sound stilted and scripted - because that's what they are.
Answer them carefully because what you say will be compared with the replies of other candidates.
Be true to your opinions, but at times it's diplomatic to leave open the possibility that particular circumstances may affect your actions. "I'd like to think that a policy of zero exclusions is right, and something to aim for, but there's a sensible saying that goes 'never say never' isn't there?"
Give the impression that you hope to learn from and respect the people you'll be working with and to bring your expertise and skills into play.
Together, you imply, you'll create something exciting and worthwhile.
The balance between the two - you as humble learner, and you as human dynamo - will vary according to the level of the post you're applying for.
A headship interviewee will give weight to both, perhaps with some emphasis on the dynamo if the school evidently needs it. A newly qualified teacher will talk up willingness to learn.
Daft questions from panel members who don't know the form need care. If necessary, ask for the question to be repeated. If it's actually improper ("What does your partner think?") then be polite but firm and address the chair. "That's not really relevant is it? I'm here because I want the job and I feel I can do it." (Though someone on the panel who knows the rules will probably hasten to intervene.) If the question's entirely incomprehensible on a second hearing, then answer a different question that you haven't been asked. No one will argue.
If you didn't understand it, then few people on the panel did.
Don't talk too much. The phrase, "He talked himself out of the job", is one of those folklore sayings in the profession. Answer each question confidently and concisely and then shut up.
Don't be drawn into filling silences with endless repetition or elaboration. If they want more they'll ask or nod encouragingly. If they fail to nod you on, or begin to lose eye contact with you, perhaps you're talking too much. Snoring is a give-away.
Make sure that you get in what you want to say. When the chair says, "Is there anything more you want to asktell us?" then don't be afraid to say, "I feel I didn't do justice to your question on..." Then give them a bit more.
Finally, mention children at some point, and smile when you do so. If you were to ask children what sort of new teacher or new head they want, the word "kind" would invariably come into their answers. They're wise in that regard. So confirm to the panel, through stories, with evidence, that kindness to children, and enjoyment of their company, is one of your key qualities.