What is classical music?
the vast majority of music/theory discussed in this thread have very little resemblance to classical music.
Short answer: Circle of Fifths - that's what drove modulation as well as how to approach modulation in classical music. I suppose you could argue essential chromaticism also influenced the chord changes to approach a key change - but typically speaking, circle of fifths.
yes for "Classical Music" the circle of fifths is the key. But film music is very very different. Most film music uses a lot of late 19th Century and Early Twentieth century techniques . The easiest way to think of the change is that you are no longer operating Under the hierarchy of a key . Chords change by different means. Quite often it's the voices themselves that dictate the changes and not the expected harmonic movement. This is Known as parsimonious voice leading . A very simple and efficient exercise is sitting at a keyboard and seeing how many triads ( maj or min ) you can play in succession by moving a finger ( or more ) only a semitone . So much of Holly wood music is made up of These progressions, Some even have names after the composers that over use them . James Newton Howard has a fav . A min chor followed by A maj a semitone below. Here the third in the Min stays on the same note and becomes a third in the Maj. Fm to EMaj for instance. This chord progression lives outside of the world of Key's and Functional Harmony that dominated music until the late 19th Century. There are Now thank goodness lot's of books on the subject:
Brian Morrell - musician, composer, lecturer, and author. Specialist in film music.
These are free !...though you could always give him something by Paypal