Make sure the candidate checks the documents as per the check list, which includes your licence and medical. Finally the emergency brief, which from my experience is often not taught or carried out. So ask the student to give you an emergecy brief as if you had never flown in a light aircraft before, so location of fire extinguisher, firsr aid kit, how to open and close the door/canopy, use of the harness, parachute (if fitted) and acitions in the event of an engine failure.
Good advice here.
I always try and make it more scenario based to help the candidates get into the groove with the flight tests, as they are often not sure what they're meant to be demonstrating, some seem to think they have to pretend they're solo and I'm not there at all.
For PPL I try and brief something like I am effectively your first passenger, and so treat me as an enthusiastic acquaintance, maybe a friend of a relative who has never been in an aircraft before, so as soon as we step air side, please brief me and treat me as such until I say otherwise. This means in flight, you don't have to pretend I'm not there. You can ask me to help look for things either on the ground or in flight, I can hold your map or pick something up, but I will not be able to offer technical advice.
For CPL I would maybe be a property dealer who wants to check out some large properties from the air, again able to look out of my side and say "yes, I can see a railway bridge crossing a dual carriageway" but nothing technical.
Along with the general brief that I will not instigate conversation to distract the candidate, but that if they feel they want to chat then that is also OK.