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.
Pawl-Shotgun!
(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.
Pawl-you liar!
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.