Film music is written specially to enhance the emotional and dramatic aspects in a film. Film music often comprises of numerous elements, such as orchestra, chorus, and sound effects in various styles. Each piece of music in a scene is called a cue. 
Nowadays, film composers use Logic Pro, Digital Performer, and Cubase to compose music with sample libraries. With the fact that technology keeps on getting better, sample libraries also keep on getting more realistic. Audiences who are not musically trained might not be able to distinguish whether the music is played by live musicians or by sample libraries. Hiring live orchestra is very expensive compared to the cost of using sample libraries. With this being set, only high budget films would be able to afford live musicians.

Theme in films:
A theme in a film is known as leitmotive, which was derived from operas in the Romantic period (19th century). There is usually a theme associated with the main character in a film. When the main character changes his/her mood, there will be a variation of a theme associated with that scene. This is common technique film composers use to make films very memorable and meaningfully enhance the emotional and dramatic aspects. Think about the movies: Star Wars, Jurassic Park, or Harry Potter.