Seating etiquette for inter-city shared-taxis will be now documented as it is quite unique to Cambodia. The vehicle of choice is of course a 90's era Toyota Camry, rebuilt multiple times in order to extend service. Taking the common Phnom Penh to Kampot journey as an example, the seating costs are currently $5 per seat.
This will seem simple and trivial at first but will no doubt result in confusion until a definition of 'seat' is given. Seats are, therefore, commonly recognized in the following arrangement:
The passenger location to the right of the driver is actually classified as 2 seats. The back area is split into 4 seats. Therefore, one can theoretically buy all the seats for $30 to reserve the whole taxi. As a single Western traveler, it is not economical to rent a whole taxi, nor is it comfortable or always physically possible to fit into a single Cambodian defined seat. Therefore, it is common to buy multiple seats, for example the whole front passenger seat or half the back.
Note that buying half the back seat does not in practice allow you to sit on half the back seat. Normally 2 local travelers will buy the other two seats but end up taking 2/3 of the back seat as is most comfortable for travelling with 3 passengers in the back. Then there is the common case of 3 people 'sharing' 2 of the back seats which results in your being squeezed as if you just bought one seat, and of course the loss of $5.
And now for the most dangerous, yet comical situation arising from your purchase multiple seats. More often than not, the additional seat to extend your comfort was not vacant to begin with so this passenger will transfer into a newly defined space left of the driver (refer to image). As may be the case, the creation of this new location may have noting to do with your multiple seat purchase but rather an experienced driver seizing a business opportunity. This space then should actually be classified as an unofficial seat for purchase at half price of a normal 'comfortable' location.