Yep, that's a bold title no doubt about it. I apologise upfront to all the composers who will find this very basic stuff but this was a real discovery for me.
I've been writing rock and pop music for many years now. I've been a fan of film music for far longer... probably dating back to the first time I saw Star Wars.
Anyway, 2 years ago I was sitting at the keyboard and playing around on a strings patch when I suddenly played a chord change that I'd just never really focused on before.
The first change was a simple Em to Cm. That was it. As soon as I heard it I realised that this was the big soundtrack sound that I'd become used to. The E melody note moving to Eb with the smooth voice leading underneath and that shared G note.
This quickly led me on to moving major triads in thirds and again hearing this delightful chromatic relationship between the two chords. My news theme uses this technique, starting in A major, moving to C major and then back to A major again.
I then began using these sounds as forms of cadence or suspension. Keeping C in the bass I would move between C and Ab major. In this manner, Ab major functions as an altered V chord (G7b9#5sus4 perhaps?) or a suspension on the I chord (Cm#5).
Anyway, eventually I decided to start looking deeper into this change and realised that I was using Chromatic Mediants ... Apparently they were used significantly in the Romantic Period and were a popular device of Jerry Goldsmith. I guess this is why they sound like film music to me.
Well, this was a big revelation to me and helped me unlock a lot of chord progressions used in films ... I think the main Fellowship theme from LotR is I -> bIII -> I (it might be I - v - I but I haven't listened to it in a while).
Of course this was a while back now and at the moment I'm exploring how much polytonality pervades film music...