Because many Romantic Period works were so long, composers tended not to use traditional forms like binary, ternary, or sonata-allegro. Instead, musical coherence was often achieved through the use of leitmotifs. A leitmotif is a short theme that is associated with a particular idea, such as a character, theme, or event. The leitmotif will reoccur whenever that idea comes up, and may undergo different variations based as the storyline calls for it.
Richard Wagner was one of the great masters of this technique. Rather than analyse one of Wagner's operas, let us look at a leitmotif from a more familiar source.
As the children practice their lightsabers, Yoda's leitmotif is presented softly in the flute in G major.
While Yoda battles the villainous Count Dooku, the theme is presented forte in the horns, interspersed with fanfare statements in the trumpet.
As Yoda gives his parting words to Luke Skywalker, the leitmotif is played somberly in the horn in A♭ major. The theme then passes to the cello in C major, before the flute gives a final warning in A-minor, based on the leitmotif in augmentation.
In a completely different movie, but also scored by John Williams, the leitmotif is heard when a child walks by in a Yoda costume on Halloween.