Hook Lines in Film Scoring
The concept of a “hook-line” – as it is called in the pop world – is to create one or several elements that stick in the memory of the listener and create something that is easily remembered and recognized once it appears again. As opposed to the classical leitmotif (which as the name says relies on a motivic hence melodic idea) the concept of a hook-line in general is broader.
Even though the name implies it to be a (melodic) line, a hook-line can also be a specific chord progression (e.g. Goldfinger), a rhythm (e.g. Terminator 2), sound colour (Harmonica in Once upon a Time in the West) or any other musical device that can be used in order to create something that “sticks out”.
Of course as also seen in the examples above, some hook-lines base on several musical factors (Goldfinger not only being the chord progression but also the melodic motif provided by the muted trumpets, Once upon a time in the west relying not only on the sound of the harmonica but also the motif).
Even though this whole principle comes from the pop world, the idea of getting your audience attached to your track by using such a device in “the orchestral world” is quite important as well. Especially as a film and media composer, you’re often expected to deliver music that sticks in the memory of your audience which is best achieved by a small idea that stands out.