Improvisations are not role-plays or simulations (although many of them can be adapted as such). The objective here is total spontaneity and improvisation. Students have no time to prepare. Their roles and situations are given to them on the spot and they have to react immediately. Generally, the less details that are given to students, the better. This allows their own imaginations to construct situations and ensures richer dynamics.
Teachers are sometimes afraid that students may not be able to cope with improvisation. In reality, it is surprising just how imaginative students will be (subject to level, of course). They can usually be relied on to give more than they are asked for. But if, on the odd occasion, an improvisation does not work, flogging a dead horse is a sure-fire way to prolong the agony.
It is often helpful to give students conflicting objectives to ensure a more difficult resolution. For example, in the case of The Hypochondriac, the Doctor should not know his patient is a hypochondriac and the patient should not know that he is consulting a particularly tight-fisted doctor. (Whether the other students themselves know of this is a decision for the teacher.)
The following ideas are just that - ideas. They can be modified, adapted, changed, rethought, distorted, simplified, made more difficult etc.
In general, begin classes with pairs to warm up and finish with groups.