Basically the way I usually see it is if you at least inform the other person on what your post will consist of. Then you have a better chance for a) feedback and critique on your actions b) possible collaboration instead of ordering outcomes that may not be preferable to a participant.

It all comes down to preference. An alternative option is for you to post your piece of the interaction. (Be careful and not to over detail, you can only describe a set of stairs so much.) Then fill the rest of the post with a short story. Did a bomb just go off in a city? News break of first contact? Others may have unique perspectives on these events.