1: You do not talk about... oh! Sorry! Wrong rules! j/k
1: (for real) Shotgun may be called only after a road trip (no matter how short) has been decided.
2: Once shotgun has been established, that person may not voluntarily cede their title. They also must, during the road trip, take on the duties of co-pilot.
3: In the event that shotgun is not able to assume or continue their duties the title passes on to the next person to call shotgun.
Notes on Hostile Takeover: It is possible to take shotgun by force. If this happens, obviously the caller of shotgun was not fit for duties and sad for them. They can get over it.