The way I look at this is:
If you are rated to fly the aeroplane and current on class, then you CAN be PIC at any point during the flight. If you are either not rated or not current then you cannot be PIC at all.
Therefore if pilot A is rated to fly in VMC and is flying the leg and pilot B has an IR but is passenger, then the minute the aeroplane enters the could then A cannot remain PIC but B *can* become PIC (which would be the most prudent course of action)....without any signatures etc. I guess the same applies to an NPPL flying to France, but should be talked about beforehand.
With a non PPL PAX, the same applies. They are not rated to fly the aeroplane so therefore can never be PIC, and pilot A remains PIC (as you have to have a PIC onboard!...in most circumstances!). There are not well defined rules to cover this, only from the viewpoint of logging the time - the pax cannot log anything, therefore Pilot A should log PIC, and it would be reasonable to assume that they are indeed PIC.
It is different for FI's as they will be PIC during a flight but of course then Pilot A cannot also be PIC but can log PUT....unless in the USA where by the FI logs "as flight instructor" and the qualified PPL as PIC.
Don't get too hung up on PIC, just agree beforehand what you are going to do. If Pilot A decides to fly in cloud then they are a prat unless they had previously agreed with pilot B that in those circumstances B would then be in command.
I've flown with non IR'd pilots that way and (depending who they are) am happy to monitor the flight from the RHS while they make could breaks, but with the understanding that if they start to get out of shape, I'll take control.
What was the original question?!
