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
- 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
- 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.)