Stage props are real objects on the stage. As a rule improvisation theater takes place without props. Usually only chairs are allowed.
Actors as props
However, there are human "props": Every fellow actor can, if necessary, step onto the stage as a prop. For example to create an atmosphere (a tree in the background, a cow chewing grass in the foreground...). There are scenes in which the prop is around till the end. But there's also the possibility to loosen up the scene and to only flit across the stage (a flog of birds, a passing car...).
There are no limits to imagination. In contrast to the passenger the prop does not intervene in the scene or influence it, unless the performers of the scene suddenly start to speak to the prop, e.g. a cow, the cow will then instantaneously turn from prop to passenger.
In the play The Thing the focus is on the actor who has to play alternating props .
However, there are a few theater groups, who have a small pool of objects backstage- especially clothing - and use them when needed.
There are plays in which real props play an important role. The differentiation from the passenger is blurred and ambiguous.