Performing improv comedy is not for everyone because it is totally unstructured. Many comedians prefer the structure of stand-up, where they know in advance what they will be doing and saying. In true improv the performers often depend on suggestions call ed out by someone in the audience for their inspiration to create scene and material.
This interactive relationship with the audience also proves that the performance is unscripted. The actors work together to create a funny scene in a completely impromptu manner. There are usually no props, but furniture, doors, gates etc are mimed into the action. The actors must then be careful to remember where they are and use them (or pretend to) when necessary.
The scene evolves by each actor defining an element as they go along. These are called offers and must be accepted and built on by the other actors. They are also referred to as endowments. They can encompass anything from naming a character, to the addition (through mime) of furniture to the scene. If they are not accepted - called blocking - then the scene suffers as it is prevented from developing.
When an offer is 'accepted' (referred to often as "Yes, and…") it is usually built on by that actor with another offer or endowment, thus the scene and/or the story is always advanced in some way. This is actually the cornerstone of improv acting. If an actor breaks out of character or deliberately blocks for a comic effect, it is called gagging. While this might be successful as far as the humor is concerned, it usually has the effect of locking the progress of the scene and so is frowned upon by many.
The mime that is used instead of props is called space object work, and the objects thus created, space objects. Actors are expected to respect this imaginary environment and must limit their actions to include it. For instance, if shot by an imaginary gun, they can't somehow survive, but must obligingly die or at least remain injured.
The challenge of playing improv is to create various characters quickly and spontaneously by the use of gestures, voice changes, accents or other devices. The comedian must be able to quickly decide on the motivation of their character and act in accordance with it. This type of acting is not for every comedian, but doing it hones your skills and makes you think quickly. Besides, it's a lot of fun and not too different to the games we threw ourselves into as children.