One important thing I have not noticed within all the excellent advice here, is a discussion about who will fly, and what the non-flying pilot will, and more importantly not do, if something unexpected happens. This has been the greatest source of challenge when I have done flight tests with a pilot, with whom I have not before flown. This pilot probably will not understand why I'm going to do, how far I'm going to do it, what I will do if something unexpected happens, and what I expect of them. Entry into a stall with a wing dropping is NOT the place to suddenly find out the other pilot's level of comfort with stalls, and you flying them! I made this error once, and never again. Now my co pilot gets a very through briefing (one hour plus), which centers on all the things I will do if something goes wrong.
I explain that I will not stop flying the aircraft at any point, unless he or she says "I have control" - then they have it, all of it! and that may not be what they are expecting. I then ask about their recent flight test and aerobatic experience. With my improved briefings, flights have been going much better, and generally I find that there is no question about who will fly at any time during the flight.
If you are properly flight testing an aircraft, you are doing a whole bunch of things that "line" pilots are trained to specifically avoid and prevent. This can create real cockpit conflict at the very worst times.
If the other pilot is doing all of the flying, expect to be told what your role will be, and stick to it. Trying to "help" fly, would be a very unwelcomed thing to do, unless asked.
Brief (or be briefed) well, fly safe, and take it slowly, according to the plan....