Let's say you have multiple lines of events going on at once in a story. Call this a multithreaded plot. Is this common in stories? How would you represent this on paper before drafting?
Multiple story lines are fairly common, and they are necessarily arising when multiple POV characters are separated for a significant length of time.
I would personally differentiate between "action threads" and "full storylines".
An "action thread" is when different characters engage in separate action and not aware of the other's action mostly because of their fast-pacing environment, which limits their observation and communication. An example: in "Star Wars", episode IV climax, Luke Skywalker, Han Solo (and also Darth Vader and Grand Moff Tarkin) each operate in his own "thread". Their limited awareness of the other's location, intention and capabilities was crucial for the overall plot.
However, these "threads" did not create separate storylines, because they serve to make the overall plot converge, not diverge. They merge into one one plotline rather quickly. For separate storylines, we need greater separation of POV characters, with greater separation of their knowledge and experience. An example of that would be separation between Frodo and Sam from Pippin and Merry near the end of "Fellowship of the Ring".
You probably need to answer a couple of questions:
- Are your threads just threads, or full storylines?
- Do you need to synchronize between those multiple threads?
Multiple storylines exists in many texts and could possibly be used in most types of stories. The only place where they probably can't happen that easily would be in a first-person narrative.
Multiple storylines can be represented in many different ways. There are two main goals you want to achieve by these representations.
- Show and verify the cohesiveness and logical flow of events of each storyline in isolation.
- Show and verify the cohesiveness and logical flow of events of the storylines in combination (the complete story).
(1) can be done with any form of list or diagram that shows events in chronological order.
I most often represent (2) as either a table or as a swimlane diagram.
The table would have one column per storyline and one row per time unit (hour, day, week, whatever granularity your story requires). Events on the same row but in different columns would then be chronologically simultaneous.
Tables and swimlane diagrams can, of course, do (1) as well, but the more storylines (or subplots) you have, the more events in the story, the higher the need for seeing each storyline in isolation.
If your story is not chronological you'll also want to view the events in "book order" and chronological order. And verify that they work in both cases.
If you need them on paper I guess you could print them, however. If you want to go the analog path, rather get some form of corkboard, create the swimlanes with yarn or something, and use index cards.
If paper is not a requirement, you could use Scrivener or other similar programs to represent the scenes in a corkboard view where they can be arranged by "label" as swimlanes (see this video on how to do that).
Tables could be created in Word using horizontal orientation and a smaller font (but a bigger screen...)