Composing music

Know what the film director wants
It's not the job of the director to do the music but he'll know what he wants in terms of atmosphere and mood and what he wants the audience to feel when they watch the film. For example, if there is meant to be a tense moment in the film the director will say that and then the composer can make music to fit that.

Be original
When watching a film the story it self has to be intriguing for the audience to be interested but if their is known to be good music in it more people will want to hear which will mean they'll have to watch too.

Experiment
If you experiment with what music you are going to put in a film it's more than likely your music will come out better because you've chosen from different music and picked the best one. Sometimes the less obvious music can be the most effective. 

Know what the story is
The music always needs to go with the story so that it makes sense. There is no point in having fast upbeat happy music in a sad scene. 

Ask for reference materials
The director will know what she/he wants so ask them for some artists or genres then you'll know you're heading in the right direction.

https://www.youtube.com/watch?v=r-L1RCtgtoE

Hans Zimmer and Christopher Nolan worked together to make a noise for "the joker" so that every time the audience heard this noise they'd know he was coming. Hans Zimmer didn't want to make a boring typical blockbuster he wanted to make something different so people would be more interested than usual. Together they did a lot of different experiments using different tools.The joker is supposed to be scary so they had to try and find a sound to create that feel. Hans Zimmer tried to create tension using razor blades and string. He then thought of using two different instruments and they were a cello and guitar. I think that the final music that they ended up playing for the joker works very well. It creates a scary and dark atmosphere which is exactly what they wanted to do.