The FAR is very clear on the subject, as it is with most all subjects. FAR 61.56(c) specifically states that no person may act as pilot in command of an aircraft unless he or she has had a flight review within the previous 24 callendar months, has passed a pilot proficiency check for a certificate, rating, or operating privilege, accomplished a phase of FAA-sponsored pilot proficiency program, or is a student pilot undergoing training (who has a current solo endorsement).
The requirement for a flight review (commonly called a "BFR", though never called by that in the FAR) has nothing to do with carrying passengers, but everything to do with acting as pilot-in-command.
The differences between acting as pilot in command, and logging pilot in command experience, are very clear.
One need not have a current flight review to act as safety pilot when acting as SIC, but only when acting as PIC.
One need not have a current flight review to act in the capacity of flight instructor, unless also required to act as PIC for the particular operation in question. (One need not have a current medical certificate, either, unless acting as PIC).
A SIC need not have a flight review, but needs to have a proficiency check every 12 months, which fills the needs of the flight review. (See FAR 61.55).
The FAR is the shortest of all the various codified regulations under the CFR, and becoming familiar with it is perhaps the easiest of any grouping of federal regulations. Compared to most any other industry, we have it easier in spades.
The FAR was intentionally made ambiguous in some respects, but only to the extent that the FAA was empowered to further define the regulation through interpretation. This was intentional, and is still done intentionally. It provides some lattitude for individual application and some degree of flexibility in adjusting the FAR to changing conditions. Outside the scope of that range of flexibility, the FAR is from time to time re-written, ammended, or changed. However, the basic text of the FAR is very straight forward, and is not difficult to follow or understand.
In any circumstance where one does not understand the FAR, or associated regulations or policies, there is ample guidance and assistance in the industry (and indeed, here on the internet) to assist in understanding it. For the few who aren't aware of the site, visit
http://www.propilot.com/doc/bbs/ for everything you ever wanted to know, or the guidance to find it, about the FAR. Good luck!!