Chapter 5 of my current WIP is a planned shift in POV to a character who seems normal and innocent at the outset but by the end of the story, will be involved in a murderous (among other things) plot in a late reveal. I want to write a chunk of his backstory in this chapter, but what I want to include happened 15-20 years prior. What would be a good way to execute this? In the chapter, he's cleaning a room, so I thought of writing a flashback, where he is remembering something that happened years ago. Another option is to introduce another character (who will also be involved in the aforementioned plot) as a surprise to the POV character and have some of the backstory come out in dialogue. But I don't want to give away too much, yet. I think bringing in the surprise character might be an obvious foreshadow. The POV fella is one of only a few characters in this chapter, so it's pretty introspective as it is, but I'm wrestling with how to tell his backstory while maintaining his perceived innocence (for now), and not keep the chapter TOO short.
There should probably be something compelling in the character's orbit regardless of what you want the reader to know. Cleaning a room can absolutely be interesting—no problem there—but I would make sure there's some subtle reason to pay attention to this character. As far as revealing a particular amount of info goes, that character could be an unreliable narrator, or simply conveniently omissive of shellshock details. You could also provide the backstory without tying it to the POV character: just scenes in prose where the POV is unnamed, or even from the perspective of another past character. It's up to you how much you reveal and when. Play with it. Surprise ghosts of Christmas past are fine if done well, but they can be hard to do well. The good thing about them is they provide easy tension, while the bad thing is the temptation for "As you know, Mark" uninspiring exposition. Depending on tone, you can also provide a red herring, where the surprise visitor seems to be indicating an old fling or a one-night-stand, but it was actually a clandestine operation to steal nuclear secrets.
In the surprise ghost scenario, I was thinking the other character could arrive and hint at past and future dealings in their seemingly normal chat. But that's not easy to execute and would be too obvious that the new character has something to do with something big. I might, instead, have him arrive at the very end of the chapter just as a point of familiarity for the POV character, but dive into their relationship at that point. OR rather, I might have that character be part of a flashback scene, only to appear at the very end of the whole book. Lots of avenues to consider there. The POV character is quiet, reserved, doesn't say much. Just goes about his duties. The other characters seem to like him fine, but none of them know his real backstory. He's the quiet coworker, the neighbor you know but know nothing about.
