Groups performing entirely unscripted comedy still rehearse regularly, which appears contradictory. What is being practised is the set of shared habits that make invention possible, rather than any content.
Nothing rehearsed will appear on stage
Rehearsal in improv does not produce material, because using a prepared scene defeats the form and audiences detect it quickly.
What is practised instead is how performers respond to each other, which is a skill that improves with repetition like any other.
The content is discarded every time, and only the reflexes remain, which is the entire point of the exercise.
Agreement has to become automatic
The foundational rule is that a performer accepts what another has established rather than contradicting it, since contradiction stops a scene dead.
Accepting an unexpected premise instantly is difficult, because the natural response to confusion is to question it.
Rehearsal makes acceptance automatic, so that a performer given something strange builds on it before hesitation becomes visible to the audience.
Structure is learned so it can be forgotten
Improvised scenes still follow shapes: something is established, something is disturbed, and the disturbance is explored to its logical end.
Performers practise these shapes deliberately until recognising them takes no conscious effort, which lets them see where a scene is going while playing it.
Without that grounding, scenes tend to accumulate incident without developing, which audiences experience as energetic but unsatisfying.
Groups develop a shared vocabulary
Troupes that work together for a long time build a common set of references and signals that let them coordinate without speaking.
A glance or a shift in position can indicate that one performer is taking the scene somewhere, and the others adjust without disrupting it.
This is why established groups outperform equally skilled strangers, and why rehearsal time is treated as an investment in the group rather than in individuals.
Practice mainly reduces fear
The largest obstacle in improvisation is the hesitation that comes from not knowing what to do, and hesitation is what audiences read as failure.
Frequent rehearsal in a low-stakes setting normalises being lost, so performers learn that a scene can be recovered from almost anywhere.
What looks like confidence on stage is largely the absence of that fear, and it is built through repetition rather than through talent for invention.