Its been three weeks since you applied and you receive the letter you've been waiting for. You've been granted an interview at a certain time on a particular date and told where the venue will be. Check your diary to make sure you're free that day. Check you're free the previous day if you have to travel a long way. Confirm by phone that you will be able to attend and if necessary ask for a few directions - write these down.
You'll probably have about a week to prepare yourself. Depending on what post you have applied for you may prepare differently. For example if its for a Part 1 post, go over your MSc project. Be able to answer the questions "why did you choose that particular topic" and "what were your conclusions". Read up on the various medical physics modalities like radiotherapy, radiation protection, MRI, nuclear medicine, diagnostic radiology as appropriate. They could very well ask you something like "what do you think is involved in radiotherapy". Take every opportunity during the interview to show how keen you are to pursue a career in medical physics and how much you know about its specialities. Don't brag though, you might get caught out.
If you're going for a Part 2 post, you would already have chosen your speciality and have a relatively in-depth knowledge of it. The trend these days is for prospective candidates to give a short (10 minute) presentation about some work they have done during Part 1. Give a clear presentation, keep it simple and know your subject. Don't exceed the time and don't show too many complex equations because they might ask you to explain them! You might want to give your talk in such a way as to guide their questions to an area you are confident in.
So, on the day itself, arrive there well in time. If you're too early you could always grab a coffee or ask to be shown round the department. Its a good idea to pre-arrange this because us physicists are usually busy (or appear to be!). When you are shown around, ask some good, scientific questions. Ask about the energy of their linacs or why they use a particular planning system for example.
Don't expect to be interviewed in the Trust Board Room or be served coffee and biscuits. Chances are it will be in a dingy little room with poor ventilation and uncomfortable chairs. After all the NHS is in financial crisis! You could be faced with 3-5 interviewers ranging from the Head of Medical Physics to Senior Physicists. They will start with some straightforward questions about your CV, so have a good explanation for that gap year. Don't say you took a year out to field-test the latest X-Box! Body language - a lot is made of this but I find that if you try to relax and sit as you would do if speaking to a colleague, you'll be fine. Don't make it look artificial and have both your palms on your knees and your shoulders perpendicular to your neck. Most physicists slouch anyway. Smile and talk at the same time - if you can't, then practice it.
Speak clearly and not too fast. You might blurt things out at the start but you'll soon relax. They'll take turns with the questions and cover everything from your academic background to the presentation you just gave to some general background stuff about your chosen speciality. At the end they are BOUND to ask you if you have any questions for them. Ask a couple, don't appear too eager to run away. Don't forget to say thanks. You can also usually reclaim you travel expenses so don't forget to drop by the admin office on your way out for an expenses form!
They will tally up their scores and exchange their impressions of you and get back to you within a few days. If you get the job you're on your way to being a Medical Physicist. Congratulations. If not, spruce up your act and try again. Don't take a 'no' as rejection. There is, after all, a worldwide shortage of qualified medical physicists and you have a good chance of getting one of those posts eventually.