There are certain regular favourites in the FAA orals.
Required documents are #1 and you
have to know this straight off.
For the IR, the #1 is the lost comms procedure (clearance, expected further clearance, then revert to filed route.... but of course the UK differs from this...).
For the CPL, a favourite is the "holding out" stuff which a CPL may or may not do before the activity becomes illegal without an AOC. This is all moot in UK airspace anyway, for a foreign CPL.
The pilot is not expected to know the FAR/AIM but needs to have a decent idea of where to look things up.
I have done the FAA PPL, IR, CPL and have generally found the oral to concentrate on real flying issues. However, if you get a b*astard examiner he will start off on obscure US airspace rules, and there was (and indeed still is, last time I heard) such a nice chap, of middle east origin, prowling for business around the UK, who ensures that you fail the checkride the 1st time so he gets two fees
I've never had a BFR (always avoided it by doing some new license/rating which counted as a BFR) but would guess that a "real pilot" instructor would do the same practical stuff. Every FAA instructor I have met does indeed fly for real.