Improvisation means that there is no fixed line and you can perform it immediately. For example, if you are waiting for a bus, you can wait for the bus yourself. You can stamp your feet and say with a sigh, "Oh, why hasn't the broken car come yet?" And so on.
Proposition performance is generally divided into groups, two or three people, sometimes five people. You will be given a topic and time to prepare, and you will discuss how to act. There must be dialogue and lines. Simple, such as "posting". That's complicated, because it involves cooperation, and you have to show your own characteristics. Don't be silly. Give it to someone else.
There are complicated ones, such as "proposing marriage". It is more difficult.
Generally speaking, more people is a simple problem, but fewer people is a difficult problem.