Thought it might be nice to give a little behind the scenes insight. I'm currently working on Nariko's Treasure episode 10. How things play out in that episode, and the ones to follow, depended a lot on some blind luck.
Before filming that segment of gameplay I had some key events I wanted to cover. For example I knew that Nariko should be a Takeda Hatamoto after the events in episode 9 cut her off from her party. Two reasons for that: 1) it befits her status as a renowned soldier 2) it allows me to demonstrate the 'freelancer' aspect of the game without first showing the long grind while you work up the ranks of the army. (Oh, and Takeda specifically because it's not too far from Mii-dera and is one of the few factions that allows promotion to the role of Hatamoto Cavalry, the most powerful troop type in the game).
I also knew that in the episode after the party was lost, some aspect of it needed to be regained in order avoid the impression that the story built up thus far was being abandoned entirely. So that meant that while fighting for Takeda's army Nariko needed to come across either Mari or Kojiro in a town.
The only town Great Lord Takeda ever goes to is Kofu, so it had to be there in order for it to seem like a surprise for Nariko (i.e. she wasn't out looking for them). Every few weeks in the game, companion characters move from town to town. That meant, in theory, all I had to do was continue being a freelancer until one of my previous pair arrived in Kofu, and hope that GLT went there during that time.
I was prepared for that to take several in-game years, and kind of wanted it to take that long as that way there would be a large disconnect between episode 9 and 10; a longer time justifies a larger change in situation for Nariko.
The final thing I knew I wanted to happen around his point in the series was a prison break, probably of GLT. Again, this was both to justify the story element of Nariko convincing GLT to let her leave his service, and to demonstrate prison breaks as a unique gameplay element.
In order to have a prison break, I needed GLT to have lost a battle JUST before I met either Kojiro or Mari, so that I could escape from captivity (the player is always captured even if their lord isn't after a lost battle), return to Kofu to learn GLT was captured and meet my companion, and then hatch the plan to free him.
This additional need made the chance of all the gameplay events I needed happening drop yet further. Yet after 9 in-game months something happened that came so close to what I wanted, and provided such great footage, that I knew I just had to change plans slightly and run with it.
It was a sequence of great events really. First while playing the battle featured in the opening scene of episode 10, I noted that the low-visibility afforded by the hilly terrain would provide a nice chance for a dramatic reveal of the scale of the Takeda army when shown for the first time. And also that I was able to keep pace with GLT for a while through the gullies on the flanks of the battle, perfect for a snappy dialogue scene.
Knowing that I was about to change my plan, I filmed Nariko walking through the Takeda army across the hill, revealing more and more of its size, providing great shots for the transition into the series introduction video. All that was useless, of course, if I couldn't get the other events to happen soon enough that they could appear in the same episode without having another janky time jump.
Of course, I was lucky. Not only did I get a siege assault and a couple of large pitched battles that were perfect for demonstrating how being a soldier worked in large clan warfare, but then I got an epic siege defense. This is an extremely rare battle type this early in the game. I was able to see out most of the fight with loads of close-calls and false-hopes, eventually ending in defeat. And GLT was captured, perfect!
If I could find a companion right then, I could do the prison break and jump into the second act of the series' storyline, especially because I had effectively shown all there was to show of being a soldier, and my story plan for act 2 needed Nariko to go independent again.
If I had gone to Kofu as I usually did, I would not have been able to exploit the great run of gameplay I'd just had. So thank the heavens for the massive bands of rebels between the site of the defeat and Kofu! Not wanting to brave them, I traveled to the Uesugi capital instead, the next nearest place. And low and behold, Kojiro was there.
I sat for a while staring at Kojiro in the inn, then shut off the game and rewrote my plan for the act 2 opening (which actually depended a lot on Mari being the one I met). All this was too good to miss, and so damned unlikely to happen that it would be a crime not to use it!
To salvage my story plan, I decided that Kojiro and Mari would still be linked in some way, so that finding Kojiro would lead to finding Mari soon after. And, in another massive stroke of luck, Mari actually showed up in Kofu after the next in-game companion position cycle, making it very easy for me to write this simplification into the story.
To support it, I left hints as to the personal relationship of Mari and Kojiro in episodes 8 and 9's dialogue, diverting from my original plan of Kojiro being vaguely hostile to both Mari and Nariko for as long as possible to give him a nice easy route for character development when he came to trust them later.
Anyway, so that's just one of many example of how random sandbox elements of the gameplay informed and molded my story plans.
Briefly, on the topic of those plans: when I began the series, I had all the events I wanted to happen in Act 1 planned out, a vague concept for act 2 with a few events I wanted, and act 3 as a single-line concept with no details at all. I got lucky for act 1, and even more so for act 2, so far at least! I plan to leave act 3 as vague as possible until I have the gameplay, as I know for sure anything I plan will go down the drain in the unpredictable late-game.
That'll do for now. Thanks for reading this, should probably get back to editing the latest cutscene with the functionally-insane Great Lord Takeda parading Nariko in front of his war council, which you'll find in its hopefully completed glory roughly 8 minutes into episode 10. Laters!