If a person calls shotgun, they automatically get that place in the vehicle. There are only a few exceptions to this rule (also called the Shotty Rule depending on where you're from), such as
~The boyfriend/girlfriend/spouse of the driver automatically has this spot unless they decline
~The driver of the vehicle has overall say if more than one person calls shotgun at once.
Under no circumstances are you allowed to break this rule.
(Gary gets in shotgun)
(long drawn out silence. Driver doesn't start vehicle)
Pawl-Gary, what the hell. Didn't you just hear me call shotgun?
Gary-I can't fit in the backseat! my legs are too long.
Driver-you just broke the shotgun rule. only the worst scum of the bottom of society's shoe would even think of doing that. Get out of my car, you're walking home.