I had one MMI interview at Dundee so I'll tell you a bit about my experience although bare in mind it will vary between unis
MMI stands for Multiple Mini Interviews and interviews are basically held in a sort of "speed dating" style. My interview consisted of 10 different stations – each of which lasted approximately 7 minutes – where I was asked to complete a task (with or without a partner/team-mate), asked some questions more like a traditional interview process or it was a role play type thing
I can't go into much more detail on a public forum on the internet as I signed a non-disclosure agreement
For prep, I'd say do the bog-standard stuff like reading through your PS, making sure you know what your answer is if they say "why medicine?" etc and think about the skills they'll be looking for in a future doctor which many of the questions may pick up on e.g. team-work, empathy, communication etc.
For the task stations talk through all your thought-processes with the interviewer so rather than immediately say "I'm going with option C because..." start with the question, identify what they're asking you to look for when making your decision and then explain why you've discounted the other options too. Also practice ethical questions - giving balanced arguments etc - and read up on some recent scientific advancements before the interview. If you make a mistake admit it! Your not trying to portray yourself as a divine god or something, show them you're willing to learn and improve yourself
A lot of what you need to do well in an MMI you'll automatically be doing for panel interviews anyway and part of the point of doing MMIs is that they through up questions you couldn't possibly have prepared for anyway so don't stress about practicising loads of problem-solving questions and stuff.
Honestly smiling, talking nicely and being friendly take you a long, long way!
Especially in the role play
You'd be surprised at the number of people who can't do just that!
Hope that helps and bonne chance!