The process is different from the vast majority of other schools, as are the selection criteria.
It is not simply an interview - it is an assessment day. By far the most important part of this day (wrt getting an offer) seems to be the team exercises. These take place outdoors and generally are a lot of fun. (They involve some problem solving with physical activity).
Most of the kids love the day and it is therefore a great "sales" tool. But it also means that those who don't get offers can feel quite crushed.
I was going to say that there is no interview, but there might be a short 1-1 chat during the assessment day, but nothing to worry about. (Just ask the registrar about that if you wish).
Their criteria are (as I perceive) mostly about "soft" factors - being a great team member, engaged and friendly etc. They seem to want pupils who will be a joy to teach and have as classmates - whether or not they are good at maths, English or NVR is of moderate importance at most. (Read that again if you have to; it really is different).
If ithe day goes well, parents and child will be invited to a joint meeting (I forget if they call it an interview) which is not part of their selection process, but an opportunity for you (the parents and the child) to ask questions. Take it! You get the offer letter just after that meeting. (All invited get an offer, so you can relax and ask honest, pointed questions!)
"Be yourself, get involved, try to enjoy it" would be my advice to a child before the assessment day.
Be aware that the process - including the acceptance deadline with a big deposit - is earlier than for most schools. Because of that, and as the outcome is quite unpredictable, it does not "work" as a back-up school.
But the flipside is that if you get the offer (and really want the school), you can skip (or reduce) the rest of uour 11+ process.