I love using flashback scenes in gaming, and often do so.
In my experience, the best way to do this is in a gaming environment where the participants see the activity as one of collaborative writing/storytelling. Take the “present” as a creative constraint for the past, in other words - when playing out a flashback, your job is to make something which enriches the present, or casts a different light on it, but does not contradict it.
Usually, this means that something like death (or other stakes which would invalidate what’s happening in the present) is off the table. Flashbacks are about establishing the details of how something happened, moreso than the excitement of wondering whether someone makes it alive through an adventurous moment. They serve a different function and operate under different constraints.
I would imagine that would be easier in play-by-post than in face-to-face play, since you can think about it carefully and frame things accordingly. Be explicit about the creative constraints, and play accordingly (“we know that the Prince escaped from the prison cell that morning… but how did it happen, and who helped him?”).
However, there are some other things you could do, to bend that basic idea a little bit:
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Accept contradictions in terms of what’s happening, and be willing, as a group, to come up with justifications later. (“Hey, we saw the Duke die in that flashback, but he’s alive now…” “Ok, I guess that means he has a body double!/I guess it means he has a means of coming back from death!”) This is unsatisfying for most groups, but could be a really fun creative challenge for others.
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Cut the scene or “fade to black” when something potentially troublesome starts happening. (“And we see the Duke hurtling off the tower into the darkness… ok, let’s end the flashback there.”)
TV shows and movies love to do this, to make us think things happened a certain way, only to reveal later that we drew the wrong conclusions from what we saw.
- Make sure that any ‘adventure’ or hazardous/dangerous situations are about something else: that something else is at stake, something important. For instance, a fight scene between some of our protagonists and an onrushing horde of monsters is all about whether the Crown Prince and the All Important Letter can manage to escape from the crypt, while the heroes defend them… once that’s established, one way or another, cut the flashback short. We don’t need to know whether the heroes survive the onslaught (because we know they did!), we only need to know whether Prince escaped in time, so there’s no need to play out the rest of the fight.
So, make sure any tense “action” scenario is ABOUT something other than the survival of the protagonists involved - that something else, which is meaningfully in question precisely because we don’t know, in the present time, how it turned out.
- Play out the flashback, and, if there are contradictions, put it down to some kind of unreliable narrator. Either it’s a question of, “…and that’s how I remember it happening, which means someone has altered my memories”, or “…and that’s what the Royals believe happened that night”, or even, “…and these are the rumours that spread in the years after the event, even though you and I know the story didn’t end there”. This can be effective if reputations or conspiracies are at stake: knowing that everyone at the monastery thinks this is how the King died, or that this is how it was recorded in a history book, can be really useful and interesting if we care about those things.