Short answer, do as much as reasonably possible with an "if [this] happens, my character would react this way" and chunk "events"/actions to paragraphs that can be retconned by your partner's next post if they don't like the way things are going.

But for that to work you should let them know beforehand that they are permitted to take a machete to your posts if they want to linger on a scene, or if they feel you forgot or skipped over something they really wanted to do.

If your posts getting slashed in a regular occurance, you may want to dial it back or take a minute OOCly to hash out where things will go and how fast.