I saw Tommee Profitt's Carol of the Bells on YouTube and was wondering if there is some theory behind the creation of the runs used he used in this arrangement. I know that these are very much comprised of patterns and the blending of the patterns creates the feel in the music. My question is "How does (should) a pianist approach an arranging project to create such an epic, film-like sound?" (I understand that it is a mix of artistic ability and taste but also theory and chord progression).
I would study some of Hans Zimmer's piano scores, specifically Inception, and The Dark Knight.
Film music relies primarily on basic theory. Such as voice leading, simple chord progressions, arpeggios, modulation (a little more advanced), repetition, musical form, etc...
If you wanted to learn more about Film scoring... I would recommend Rick Beato's tutorials on Film Scoring. Good Luck!