1. STRUCTURE
1.1. A quickly changing structure can sound fragmented, create unease, or be exciting...
2. Tonal (key) relationships in Baroque (and some extent Classical) music were an important source of contrasts in eras that had less textural and rhythmic contrast than the current day.
3. EXPRESSIVE reasons
3.1. HARMONY
3.1.1. Dissonance can create a mood of unease, tension or even fear
3.1.2. Chromatic chords in the Classical period (Neo 6th / Ger 6th) can be dramatic / examples of word painting to expressive
3.2. MELODY
3.2.1. Melodies limited to low registers sound sombre
3.3. TONALITY
3.3.1. Major keys can be bright, joyful, exciting...
3.3.2. Minor keys can be sombre, dark, mournful...
3.3.3. Ambiguity in tonality can create tension, mystery, menace...
3.3.3.1. Modal music evokes folk music / ethnicity
3.4. RTM
3.4.1. Dotted rhythms can sound playful
3.4.2. Constant (quavers) can sound relentless, create a feeling of chase...
3.5. SONORITY
3.5.1. Prominent trumpets in film music can sound heroic
3.5.2. Low brass can sound menacing
3.5.3. Pizzicato strings can sound delicate, magical, playful...
3.6. TEXTURE
3.6.1. Homophony can sound grand and powerful
3.6.2. A pedal can create stability, a sense of grandeur...in a minor key, a sense of menace, tension or foreboding...
4. CONTEXTURAL reason
4.1. Reasons relating to the time and place the music was writen in and the intending performance circumstances
4.2. Bach wrote for a 3-tier orchestra (concertino, ripieno and continuo) to create the dialogue between parts in the texture that was highly desirable in the Baroque period
4.3. John William's music uses forces that remind us of the Romantic era, used in early romantic films but less so in science fiction. By doing so he is able to crate a wide expressive range.
5. MUSICAL reasons
5.1. HARMONY
5.1.1. Circle of 5ths helps establish tonality
5.1.2. Chromatic chords add contrast and variety to functional harmony.
5.1.3. Passing chromaticism, in Classical music, adds colour to the harmony, often making the harmony 'lighter'.
5.2. TEXTURE
5.2.1. A pedal note helps establish tonality
5.2.2. Polyphony is an elaborate and ornate texture
5.2.3. Changes in texture can create contrast and variety in the structure
5.3. TONALITY
5.3.1. Modulations in Sonata Form help characterise the structure
5.3.2. Modulations can help create contrast, especially in Baroque and Classical
5.3.2.1. Classical and Baroque music, in diatonic keys, helped focus interest on the melody
5.3.3. Passing chromaticism, in Classical music, adds colour to tonality, often making the music 'lighter'.