I have read the topic from some 5-6 years ago (clicky) but I am in a slightly different boat than where the OPer of this topic.
I am looking to write a symphony in the sense of a soundtrack, yet more as the script of a story than a soundtrack itself. Sort-of like Fantasia by Disney, but more a single story with voices than a orchestral narrative to many stories. I want to write the symphony so that an instrument is taking the place of someone speaking. (For example, I have a clarinet play instead of Leonardo de Caprio speak the main character).
I have a fundamental outline of a story which will be between 90-200 minutes when read aloud. This story has 1 main character, 3-7 supporting characters, 7-20 additional/rotating characters plus various scenes with loud and vociferous crowds.
I understand basic music theory. I've been in two choir classes, played the drums and violin each for a year. I can read music, identifying the parts of the staff, each note, the various "commands" (da cota, ds da cota, repeat, crescendos, decrescendos, etc), etc etc. I understand there are ranges for each instrument, that some are able to play multiple notes/chords at once while others are limited to single notes. Yet, outside of the basics, I do not have terribly much music background.
In addition to listening to the various symphonies, reading the assorted books, locating study scores to "read" while listening, I foresee that I will need to: identify the characters and bg events in each scene write out the script of the story identify the emotion being displayed in each statement/line identify the bg ambience in each scene as the scene progresses begin searching for solo scores of various instruments to begin identifying which instrument I desire for each character
I also recognize I will likely recruit someone down the road to help me compose and arrange this as a whole. But until that point, is there anything more I need to be looking at or keeping in mind?