If it is were me, I’d use a tool like Chronicler to outline my story, choices, and conditions first, fill in the story text, then let the program write the code for me! 
Shameless plugging aside, I definitely recommend writing the code first. (after you have the outline and character sheets done of course.) Then worry about filling in the story. My thought process is that if you have some mechanic that is critical to the story/gameplay, then it is best to get it done first. If you were to spend hours writing the text, only to find that a certain bit of code isn’t possible, (or extremely difficult to insert into all that text) then all that time spent writing was wasted.
Take Tin Star for example: one of it’s most endearing traits was the recurring town sequences wherein you could talk to the townsfolk and improve your character in various ways. If the author hadn’t taken the time to have that system in place first, it would have dramatically affected the rest of the game, in writing as well as mechanics.
(I recall reading this in some article awhile back. Can’t seem to find it now though 
)
Hope this helps!