I believe that first use the same interview questions across all companies (may be wrong but it may point you in the right direction).
Therefore a qualified driver may be asked all the usual rubbish about why you want to move company, what you know about first/FGW etc and then questions along the lines of (I'm not going to give the exact questions or scenarios I was asked but a general idea)...
-what makes a 'professional' driver
-what would you do if driving along under greens and you get an unsolicited brake application
-what would your actions be if someone reported smoke coming from your train to you
Hey may probe further into answers to these, throw more in depending on how you answer etc to get down to things like rule book modules in the 'M' range, emergency protection, assistance protection etc.
They may also ask about any challenges you may face moving company, how you 'get yourself back I'm the cab' if tired/bored at the end of a long shift when your mind is wandering, ask about any incidents, anything you personally do to avoid incidents (RTC etc).
That is sort of what FCC asked me as a qualified driver anyway a few years back.
Obviously just brush up on rules, traction and PDP for your company, if there are any rule book modules you currently don't know but will need for FGW (AC electrification/multiple units etc ) then read through those so you have some idea and reasearch the company. Also see what you can find out about the traction and routes (don't need to sign them yourself obviously but any notable features, comparisons to stuff you currently sign etc).