If you follow any discussions of film music on the Net for long, sooner or later you'll see the word leitmotif, and unless you're already familiar with the term from music history, you probably wonder what it means. That's natural, because even within the film music fandom community, the leitmotif idea tends be misunderstood.
Stripped to its most basic terms, a leitmotif is a melody, or a melodic figure, that in a film score is associated with a character or object or idea or whatever. A perfect example is the Imperial March from The Empire Strikes Back, which is almost uniformly associated with Darth Vader. Thus, a leitmotif score is simply a film score that makes extensive use of leitmotifs.
Why do composers use leitmotifs? Because they lend unity and coherence and richness to scores. A skilled composer can use his motifs to play off one another, musically, as characters do; and the recurrence of themes and motifs as a score progresses enhances the audience's familiarity with a film and its characters. Not all film composers use the leitmotif tachnique; Jerry Goldsmith, for example, tends to employ a "theme and variations" approach in which a single theme or small group of themes wends its way through an entire score, along with atmospheric music that reflects Goldsmith's main classical influences, Ravel and Stravinsky.
More than that, a skilled composer who puts thought into the creation of his motifs will create them in such a way that the motifs themselves are interrelated in a proper way. Thus, for example, virtually all of the major themes in the Star Wars films that are associated with "the good guys" begin with wide intervals: the main theme (Luke's theme) opens with a fifth; Leia's theme and the Love theme from TESB open with a major sixth; the "Brother and Sister" theme from Return of the Jedi begins with a perfect fourth; the Love Theme from Attack of the Clones with a minor sixth. (Of course, John Williams historically seems to love the fifth, which is also the basis of his Superman and ET themes.)
Less obvious are some other interrelations of the Star Wars themes. The Imperial March (Vader's theme) and Yoda's Theme don't seem all that similar, but both follow the same rough melodic shape in their opening phrase (hum them both and you'll hear it), and the AOTC love theme is a minor-key variation of the Star Wars Main theme. The two major themes that don't fit into any particular mold are Ben's Theme (or the "Force Theme"), and the Emperor's Theme. These are pretty iconic, and are nearly always heard in pretty close to their original form. And there are loads of smaller themes that are not heard nearly as often – Jabba's Theme, Lando's Theme (only heard in TESB), and others.
How does Williams employ all these themes? This brings me, actually, to why I refer above to the common misunderstanding of the leitmotif technique. Most people assume that leitmotifs exist to basically "mirror" whatever or whomever happens to be onscreen; thus, when Luke Skywalker does something heroic, we should hear Luke's Theme; when Darth Vader is being evil, we should hear his theme, and so on. It doesn't really work this way, though: leitmotifs aren't just musical tags to be sounded at any point when that motif's referent is acting. This is what I call the "Easy View of Leitmotifs", the idea that they are the musical equivalent of nametags at a convention, or perhaps those little tags on thumbtacks beside each specimen in the dead butterfly exhibit at the Museum of Natural History.
This is false. Leitmotifs are musical ideas, first and foremost; in fact, the great "inventor" of leitmotif, Richard Wagner, referred to them as "melodic moments of feeling". A skilled film composer will use leitmotifs to create a tapestry of emotional music, as opposed to a grab-bag of disparate themes that happen to "mickey mouse" the action onscreen. Thus, a common complaint filmscore fans make of leitmotif-based scores – that the motif is played when its associated character is nowhere on screen – is often misplaced. A recent thread on a message board, for example, criticizes the end of John Williams's Attack of the Clones score for playing the Imperial March (Vader's Theme) when the clone armies are shown in regiment, since Darth Vader doesn't even exist yet. But the March isn't strictly associated with Vader, but also with the Empire itself, and what Williams is musically conveying is that this moment is the true birth of the Empire, even though Palpatine has not yet declared himself Emperor and Anakin is still a Jedi.
Problems can arise with the leitmotif technique. Comparing the use of motifs in the Star Wars scores with the Lord of the Rings scores, I find that the latter are more unified in their conception than the former. This really isn't the fault of John Williams, though – he didn't have the luxury, as Howard Shore did, of knowing the entire story from the outset. He didn't know how important Darth Vader would become to the story, for instance, which is why there is no trace of the Imperial March in A New Hope (although he does base the Imperial motif from that film on the same interval – the minor third – from which he derives the "Rebel Spaceship Fanfare", a prominent motif throughout the original trilogy). Thus Shore was able to do a lot more "pre-planning" of his motifs, whereas Williams often finds himself "retrofitting" new motifs to fall into place alongside the old ones. Of course, he's Williams, so he does so brilliantly. But it's still a challenge and doesn't quite always work perfectly.
How important is all this to the enjoyment of a filmscore composed in this way? Well, what matters most of all in a filmscore is how it enhances, or illuminates, the emotional fabric of a film. But if you're after a deeper understanding of a score, it helps to consider how the motifs and themes are related and why they are constructed the way they are.