I first experimented with parallel story structure when I wrote Plague of Shadows for Paizo’s Pathfinder series. In that book, I’d occasionally flash back in time fifteen years for vignettes that slowly revealed background relationships illuminating why the characters acted toward each other the way they did in the present.
It wasn’t a NEW technique, for I’ve seen it done elsewhere, and you probably have as well (especially in Scott Lynch’s Gentleman Bastard books). But it was new to me.
In my new series I’m attempting to use it in a far more complex manner, for approximately half of each book (thought not all in one chunk). Characters who are dead or missing in the present are point of view characters in the past, and they are searching for explanation of mysteries that we see characters in the present discovering (all while dealing with the aftermath of actions taken by both themselves and these other POV characters long ago).
I switch back and forth between the past and present — part one is in the present, part two in the past, part three in the present, etc. It’s tricky. I don’t think I’ve ever had so many balls in the air at once. I’m striving for some very subtle effects. For instance, one small way I’m trying to ratchet tension is by the reader knowing that one (hopefully interesting) character will die a day after she’s promoted, and that promotion gets closer and closer as we get to know her better. And then, the day when she’s supposed to be promoted, she’s injured and can’t attend the ceremony — how much longer DOES she have? I’m hopeful that will create tension.
I’ve also set up rules for myself. For instance, I don’t use anyone as a point of view character until they’ve been either introduced through the eyes of a POV character, or discussed by them several times. Obviously the first POV character is new to readers, but after him there should only be reactions along the lines of “Oh, I’ve met this character; it will be interesting to see what he or she is like” or, hopefully, “cool — they’ve been talking about this person from the past and now I get to see why they thought she was so interesting/deadly/competent.”
Right now almost all of my creative energy is focused upon revision of the first book, so I’m tightening the through lines, cutting dialogue and sharpening prose. I can’t be sure it’s all working, but I’m enjoying it, and in my experience that’s always a good sign.