Better living through reliability.
Narrative. Narrative. Narrative. Going into a postmortem meeting, you should have a plan for what to focus the discussion on and what to avoid.
Postmortems are complicated. They're complicated because the systems we use are complicated. On top of that, people are complicated and our organizations are complicated. A postmortem has narrative elements to it, but the postmortem meeting should definitely have a narrative to it. It's up to the postmortem presenter (usually the author) to define and drive that narrative in the postmortem meeting.
Prior to the meeting, you should have a crisp summary of what went wrong, impact, and resolution. And you should be able to quickly explain this in the first few minutes of the meeting. If an executive asks a clarifying question about these facts late in the meeting, then they've been in the weeds far too long and you messed up.
Next presented are hopefully a bunch of straightforward facts, conclusions, and action items that are unobjectionable and everyone already agrees on. Quickly getting through this list of undisputed items is key to leave time for disputed or unexpected items later in the meeting. Hopefully there are simple/common best-practice ways to address these issues that are already captured in the action items, else you'll need to allocate time to discuss fixes. This is also the section of the meeting that can drag on if folks get into the weeds. Be mindful of everyone's time and keep everyone focused.
The rest of the postmortem meeting should dig into unknown causes, unresolved fixes, and what action item gaps are still missing. This requires clearly articulating to the room what the known unresolved issues are, focusing and soliciting discussion around those unresolved issues, helping the room come to some consensus of the path forward, and distilling all that into action items during the meeting.
Hopefully all the possible unknown or unresolved things to be addressed in the meeting are already clearly called out in the doc prior to the meeting. If the doc was sent out far enough in advance of the meeting, most gaps will already be discovered. But fostering and enabling discussion for gaps is still critical in the meeting. Not everyone want to speak up in a group or ask a "dumb" question. It's your job running the meeting to ensure you make a safe enough space for people to come forward and voice concerns.
In theory all action items and discussion happen prior to the meeting. But people are busy, writing feedback in docs is a low bandwidth form of communication, and sometimes we collectively miss things that we notice in a group.
Somtimes you simply don't have access to the right people to answer certain postmortem questions before the meeting. Maybe they're an exec that you never talk to, maybe they are on another team and you didn't reach out to them, maybe they were out of office for several days. When you have the right people in the room, then hopefully you can quickly iterate and sketch out solutions these lingering issues. Sometimes people come up with great last minute solutions that no one yet suggested, or sometimes during the discussion you have your own epiphany or clarity about a better solution.