This game combines improvisation with careful observation, and so makes a great rehearsal tool for serious improv performers who need to be able to see and imitate easily. Plus, it's lots of fun.
Set up four chairs to resemble the front and back seats of a car.
The first three people get in the car, leaving the rear passenger-side seat empty.
The three people in the car invent a scenario to explain their traveling together, and mime driving along, improvising a conversation, etc.
The next person in line enters the scene as a hitchhiker with a fairly clearly defined character.
The people in the car must stop to pick up the hitchhiker, but they improvise how politely they do it, etc., based on their characters.
Once the hitchhiker is in the car and a four-way conversation begun, everyone in the car begins to pick up the hitchhiker's personality and mannerisms. (For instance, if the hitchhiker is paranoid, soon everyone is paranoid. If the hitchhiker is drunk, soon everyone is drunk. If the hitchhiker is excessively cheerful, soon everyone is etc.)
Once everyone has fully taken on the new personality, the driver leaves the scene and everyone moves over one seat, so that the front passenger becomes the driver, the driver's-side rear passenger becomes the front passenger, etc.
Repeat with a new hitchhiker, who has a different personality. (Until they have picked up the new person, the three in the car continue to play the first hitchhiker's personality.)