It's not just choice; you have to learn them and interiorize them and they're subjective. Grant Morrison and Alan Moore and Neil Gaiman can write incredibly confusing, irrational, impossible stories that nevertheless are plausible and gripping and immersive. This took them decades of experience. Your beginner fanfic writer, no matter how well-intentioned and studious, will fail on some fundamental level. Check out EY's earliest fiction out there; it's pretty damn terrible.