Thank you, Smillew,
The impro reference made it clear.
I recall a few such exercises I was doing at some point. I always felt a bit nervous waiting to see which actor I'll have to follow on our improvisation exercises. I dreaded joining in certain persons because we had such different personalities and acting styles.
So I see it has to be some chemistry in writing impro too (aside from the willingness to leave some doors open) and the two of you seem to have it.
Looking forward to reading the next one!