When a writer has a big, complicated novel with lots of subplots and plot arcs that need to weave around each other, there are two main things he/she needs to do: 1) keep track of all the things that are going on offstage and in different plot arcs than whichever one is currently at the front and in focus, 2) making sure you don’t stay solely focused on a single plot thread for too long (because that makes the book feel lumpy and unevenly paced, or, in extreme cases, like a series of short stories strung together).
To write a novel with lots of subplots and arcs that need to intertwine, a writer needs to 1) keep track of what’s going on offstage in all the threads that aren’t currently at the front and in focus, and 2) make sure he/she doesn’t focus on only one plot thread at a time for too long (because that makes for lumpy, uneven pacing, and in extreme cases, for readers who’ve forgotten half of the important things going on).
Basically, keeping track means taking notes and updating them regularly. There’s no other way to do it, really, unless you have an eidetic memory. Notes can be done in advance or after-the-fact, or both at once (if you check in at the end of every scene, you’re also checking in before the beginning of every scene that comes next). I’m a check-in-after/before-every-scene writer; before I start in on a scene, I want to have some idea what’s going on in each of my subplots. And since my scenes rarely go quite the way I’d planned, I need to look at them as soon as I’m done writing them and figure out how what did happen onstage is going to affect the way all the offstage plots are developing, which in turn will affect what the next scene is and what happens in it.
One of the ways I keep track is with a calendar, because most of my stories are told in chronological order over a period of weeks or longer. I usually set up a one-month template in Excel (because it gives me more control than a real-life calendar program) and as I write each scene, I log in what happened, time and viewpoint (if those are relevant or not obvious), place, and any important events (e.g. “G and J picnic in Central Park; Uncle W interrupts; J loses necklace”). This gives me a picture of what my characters actually have done (as opposed to what I’d planned) and when, and whether it’s plausible to have G foil the villain’s bank robbery in Paris at 3 p.m. when he was picnicing in New York with J at 11 a.m.
For the Frontier Magic books, I had a list of what-happened by year and how old the main character was (because those three books cover nearly twenty years). I also had a chapter summary at the start of each draft chapter that said something like “1843 – Eff is four/five; family leaves for Mill City late summer” so that if I had to move scenes or bits of narrative around, I’d have some idea whether I’d have to check all the age and date references or not (if they went in the next chapter, “1843 – Eff is five; arrival and settling in to new house” then no; if the bits moved three chapters forward to “1849 – Eff is 10; Eff is 11; McNeil Expedition leaves town,” then definitely yes.)
Making sure you don’t focus on just one plot thread at a time again requires awareness, first of all. Once you know it’s something you need to do, there are two basic techniques for doing it: first, you balance the scenes, interleaving the various plot threads so you don’t have eight action scenes in a row and then eight romantic ones; second, you incorporate more than one plot thread into the same scene as often as possible.
One technique for balancing the focus is making a color-coded scene list. Again, you can do this before you start writing, build it as you write, or use it as a tool for analyzing your first draft once it’s complete. List the scenes in the order that they appear in your manuscript (“G and J picnic” “L kidnapped” “Q steals secret bomb plans”) and then color code them according to whether they’re part of the main action plot, the sidekick’s romance, the annoying little brother’s subplot, etc. If things are well interwoven, the list will end up looking like a rainbow, with colors changing quickly; if not, it’ll look like large blocks of color stacked one on top of another with little overlap.
When I do this, I get a zillion different colors of Post-It-Notes and assign them to the different plotlines, then lay them out in order on the dining room table. This lets me move scenes around easily once I’ve noticed that I have six bright green Post-Its in a row and then no green ones at all for the next 20 scenes. Using Post-Its also means they stick to the table so the cats can’t scatter them all over. Also, I can stick two different-colored Post-Its together when I realize that I can have the kidnapping happen during the romantic picnic and get a scene that’s a two-fer. I also look for scenes that I can delete entirely. You don’t actually have to dramatize everything, just because you can; it’s OK sometimes to say “After three hours of shopping, they finally had all the parts for the bomb” instead of writing three scenes where they stop at different hardware stores to buy what they need.
Incorporating more than one plot thread into the same scene uses exactly the same sort of skills that writers use to put plot, characterization, and background into the same scene, or dialog, action, and summary. There will be some scenes that can only do one thing – it doesn’t make sense for the hero to stop in the middle of the crucial fight with the villain to worry about his grandmother’s illness, for instance – but a lot of the time, you can work two or more subplots together (as in the action-kidnapping interrupting the romantic-picnic mentioned above).
This multiple-plotlines-per-scene technique is particularly painless when two or more characters are in a position to talk for a while. Whether it’s a tea party scene or two characters talking on the bus or at the water cooler, conversations can be full of gossip that covers several other characters’ romances or financial problems (and their associated plotlines) in addition to whatever planning/plotting/clues the scene was originally thought up to provide. The trick here is usually to pick one main subject of conversation (presumably whatever the point/plotline the scene is supposed to focus on), and then look for places where the characters would naturally get off course and talk a bit about seeing George at the bar with a shady-looking character last night, or just how Jin is supporting her shopping addiction. One can also occasionally have such a scene interrupted by a phone call, letter, or the arrival of someone new with a message, which gives the illusion of bringing some of the offstage developments onstage temporarily.
Also, do remember that for a lot of writers, doing this kind of in-depth scene-balancing analysis is something that’s only necessary when they’re stuck or when there’s a problem that they can’t put their finger on. I don’t haul out the Post-Its for every book I write, and even when I do, I don’t always haul them out in advance.