: How to turn a single narrative into a branching one? I'm currently writing a story which started out as a short story but kept growing, until I discovered it would work well as a visual novel.

I answered a very similar question recently here: How can I convert a linear narrative into a branching narrative?

I tried to develop a step-by-step process through which one could convert a linear story, like yours, into a narrative with many paths. Even if you've already started on that process I think it could be helpful to you!

Here are the parts most relevant to your question:

Think of it as a braided river rather than a branching tree

Imagine a braided river channel. You can put a rubber duck in at a single point and follow its progress down the river. It has many different possible paths, but it is not locked into a unique path by one early decision. Reliably at many points those paths converge. As it approaches its destination it may have a delta with a few different branches, and only then does its final path to the end of the river become fixed.

Pick-your-path narratives benefit from having a similar structure. Your protagonist starts at a particular scenario and is offered a few decisions, and they come out at one of several possible endings, but along the way they can get to specific scenes from a variety of previous paths. You do not have to write 3^15 different stories, where at every decision you create new paths and scenarios, because you can write decision-making options at several different scenes that feed into a single new scene.

Since you're adapting a linear story, I would suggest doing the following:

Decide what the starting point is.
Consider how you want to handle the novel's ending. (@CatofHearts, you have this part figured out, but if anyone else reading this isn't sure about it, see below.)
Identify any scenes or events in the book you absolutely want your protagonist to experience. These can include character- and world-building moments as well as those that serve the plot. (These will be confluence points for all possible narratives. The fewer you have, the more your paths can diverge from that in the book.)
Map these out in a visual diagram or flow chart. (You don't necessarily have to decide what all of the endings are yet, but include all the ideas you have now.)

Now you can fill in the gaps with as many meandering and braided shorter paths as you'd like. To make the most of the source material, I would suggest trying the following:

Identify any scenes or events in the book you would like your
protagonist to experience that are not absolutely essential to
reaching the points identified in #3 . See if you can create or
identify various smaller, independent sequences of scenes from these
connected by 1, 2 or 3 decisions made in the book. They can overlap. Map these out separately from your
core chart, and include the decisions that connect them. (I.e.,
begin to make small braids that can serve as building blocks.)
Identify all of the illustrative interlude scenes that serve
character- or world building but don't involve decisions that affect
the plot. Hang onto these.

Now you can start creating your own content:

Go back to #5 . For each of the decisions you identified, create 1-3
alternative choices the protagonist could make. Can these lead to
other scenes that already exist? If yes, great! Keep them. If not,
try to create new scenes with decisions to make that can lead back
into existing scenes, especially the ones identified in #3 .
Start putting these smaller braids into the core map in the most
obvious locations. Connect them to the confluence points with new
decisions. Remember that because this is choose your own adventure,
these aren't locked into a chronological order. For example, even if
most choices leading to the braid happen before a specific
confluence point, other paths can conceivably go through these from
after reaching that confluence point (see #3). You can prevent loops
with conditional choices, e.g. if the protagonist has already met a
troll at a confluence point, there is a fallen tree blocking the one
path that would lead back to the confluence point with the troll.
Make sure that none of the paths skip the essential confluence
points.
Hammer out your endings, and make decision chains that bring in
existing braids or entirely new braids that get the protagonist from
the final confluence point to each ending. You may be working backward to make this happen.
If there are any loose ends (decisions that don't lead to an
existing scene), you can start filling those in with original
content. Go back to the scenes identified in #6 for inspiration--
see if you can include these in the path and create related
decisions or actions that can can lead to one or more existing
scenes. (Again, make sure that none of the paths skip the essential
confluence points.) If some just don't work, prune them.
If you have any leftovers from #6 that you still want to use, you
can work these into existing scenes or make them brief interludes
without decisions to make.

(Added back in later, for the benefit of those who haven't written all their endings yet...)

Determining your endings

You'll want to decide from the beginning roughly how many ending options you want. This may hinge on the way the novel is structured and the sort of conclusion it reaches. Some possible structures:

Does your protagonist have a single literal or conceptual destination
to reach by the end of the book? (e.g., Mario adventures)
Does your protagonist have a goal at which they and their allies can
either only succeed or fail? (e.g., saving the planet, restoring the
Republic)
Is this a mystery story wherein several possible solutions
are suggested or implied before a final reveal? (e.g., whodunit)
Is the conclusion less important than the journey? (e.g., speculative
fiction, Catcher in the Rye)

Once you've thought about those you can identify which of the following sets of conclusions you want:

A finish line; your protagonist either reaches it or dies along the
way, only to start over/back up and choose a more successful path.
(e.g., Mario adventures.)
A binary outcome of success or failure of an ultimate goal. There could be two
outcomes, or there could be multiple, not terribly significant
variations of success or of failure, perhaps plus one divergence from this. (e.g., the villain is defeated
in one of three ways, only one of which is canon for the source
novel, or the world is destroyed by a nuclear war before the pandemic
can wipe everyone out.)
Many possible, unrelated endings. (e.g., different characters could
be the perpetrator of the crime, or your character can have happy/sad
endings with different final destinations, careers, partners, powers
or ignominious failures in life.)

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It's hard to answer this question without being overly broad.

Length and Structure

The blogger These Heterogenous Tasks has been writing his analysis on specific CYOA books. I wrote a summary of his article Standard Patterns in Choice Based Games.

Certain branching structures lend themselves to different end-goals and game styles. Gauntlet stories have 1 true ending with many sudden deaths. Branch and Bottleneck offer parallel paths that merge back into the main plot. Quest stories allow the larger narrative to be non-linear by requiring a series of smaller quests to be completed individually, in any order.

Visual novels can be any length, but a major factor in how the branching narrative is structured is whether or not someone is expected to re-read it to experience the different endings, or if their ending is the one true ending for them.

A long story is not going to be re-read, every ending needs to be narratively satisfying. Unless you are writing a sudden death gauntlet and the reader can back up from failure to try a choice again, they will be stuck with the choices they've made all the way to the end. They may not care, or remember, arbitrary choices they made early on, especially if you've withheld information about the consequences.

Conversely, the shorter the story the more often it can be re-read. Quests and Dating Sims can be thought of as many short stories contained within a frame story. The concept of reading several in a row is built in, and individual choices will feel less life-and-death important.

Expanding your existing story

Under the hood every branching narrative is unique. There are conventions, and a story might set up its own consistent "rules", but the forking and merging are ultimately created in situ to fit the specific story. There will be very obvious life-choices, but also trap doors that send you to another part of the story, and locks that will need a key before they can be opened. Your options will depend on the software you are using, the story you are telling, and your skill as an author.

Here are some ideas:

Create parallel subplots as in a Branch and Bottleneck
structure.
Each "scene" is a puzzle that needs to be "solved"
spread a Quest narrative across an "open map" the reader needs to
explore
Deconstruct your novel into the important narrative beats and design
major branches only around these moments. Other choices will be
almost decorative, to effect the atmosphere and tone.
Decide the narrative POV. Are readers roleplaying via a game avatar (dating sim)? Exploring as themselves (You awake with amnesia…)? Or are they guiding a third-person protagonist?
If your story has a moral theme with only one "true" ending, who is
being punished for getting it wrong, the protagonist or the reader?
Are you providing the reader with everything they need to
make the right decisions, or is there an element of randomness and
chance? If they make a wrong decision would they know it, and can
they get back on the right track?

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You have your main path already, which is a good start. What I would do from there is identify the main choices that the protagonist makes - the things they say or do that drive the plot in that specific direction - and then turn those into the branching points. For example, if your true ending involves the hero turning down the villain's "we can rule together" offer and kicking his ass, make that offer a decision point and give the player the opportunity to accept. The hero ruling alongside the villain can then be one of your alternate endings.

Regarding "what kinds of choices" there are, visual novels generally make a distinction between action choices and dialogue choices - i.e., what to do vs. what to say.

If you're choosing an action, you'll set at least one event flag, and those flags will then drive the story in a specific direction. Actions generally have an immediate (though not necessarily major or long-lasting) effect on how the story plays out.
If you're choosing a line of dialogue, you'll usually affect a variable that represents how much the character you're speaking to likes/dislikes you. That variable, in turn, will affect how the character behaves towards you later in the game. These dialogue choices will always lead to slightly different conversations, but they may not have any actual effect on the overall storyline until later on.

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