Sweeping scores that would seem completely over the top for a pop song have pride of place in films. But how is a modern movie soundtrack made? And what makes a good movie soundtrack?
Let's take a look...
The Musical Process
The process of putting music to film follows three distinct steps...

For conventional movies, the musical process begins with a rough cut of the film - the composer will have a chat with the director, going through the film and noting down the different styles of music and ideas that would suit the different scenes.
Occasionally, a director will edit the film to temporary music - existing pieces that give an idea for the feel the director wants. This can annoy some composers, since directors can become so attached to the temporary music that they discard the music the composers have written to replace it, such as in Peter Jackson's King Kong.
Scoring
The next step is actually scoring the film - while some composers still work with the traditional paper notation, the vast majority take to their computers and compose with MIDI.
These are often affectionately called "MIDI Mockups", which are how the composer presents his compositions to the director before the final step.
Sometimes at this step, directors are so taken with the music that they re-edit the movie to fit it better. Probably the most famous example of this is when Steven Spielberg gave composer John Williams free reign over the music for the chase scene in ET, then the film was edited in time to the music.
Performance
Once it's all orchestrated, the music will be recorded by an orchestra. Usually, the composer will conduct the orchestra. This is generally done in a room with a large screen playing the movie, to a click track so the orchestra can stay in time with it.
After the recording session, the music is ready for integration with the actual film, where it'll be mixed and mastered.
Usually the performers don't get credited, but lately they've been creeping into the credits under the name "Hollywood Studio Symphony" - an umbrella term that covers all the session musicians they employed for that recording.
What makes good film music great?
So that's how it's done, but what makes for a great movie soundtrack? Why do some of them just accentuate the film and others have you humming them every time you see the DVD on a shelf somewhere for the rest of your life?

These are the parts of the music that you relate to the film - things that reflect the feeling of the movie. For example, in the Harry Potter films you'll hear that same catchy theme music again and again that implies something magical and innocent, yet a little bit dark. The Terminator theme music with the mechanical drum beat. The magnificent theme for Jurassic park that reflects the impressiveness of that first sighting of the island. The scary 3-note motif in "Signs" when the aliens are just around the corner...
So what makes a great soundtrack? A great film soundtrack contains music that can reflect what is on the screen in a memorable way. Whether it's harsh electronics, immense orchestras or a combination of the two, film music is capable of being as popular as any chart hit and as epic as any classical composition... and has the benefit of a ready-made music video to accompany it!