In the real world, meaning studios bigger than ours, the first step a composer takes with a director is to "spot" the music. That is, to look at the locked picture and decide where music is needed. You look at things in the picture which want to be "marked". That's where you see a character's look, or what they're looking at, or a moment, which needs a little musical "mark" to indicate it.

It might seem strange, but even though I come from a music background I don't really enjoy the work of spotting the music tracks.


The modern composer ends up having to be a sound mixer too. And a sound recordist. And someone who's good at making orchestral-sounding scores "in the box". Except for fairly rarefied composers, and big-name composers, a composer seldom gets to work with an orchestra.
Back in the day every composer needed to have the Gigasampler application with a whole bunch of sounds. Nowadays there are a lot of applications which have huge libraries of orchestral sounds. My advice is to get one of these programs and get some sounds and really learn how to use it. You'll likely find that some things work better than others. Maybe your tremolo strings are to die for. So? You use a lot of tasty tremolo strings.
The most counterintuitive thing to a composer about music for picture is the hole you have to make in the middle of the music. You have to leave a hole, right where the melody would usually go, for the dialog. Remember, that in the movies, dialog is king. If your music collides with the dialog, somebody (the "re-recording-mixer") will turn down your music in favor of the dialog.
So if you are scoring a dialog scene, you're going to have a lot of very top end and very bottom end instruments.
What's worse is that if I were mixing your music and it was stepping on dialog, I'd take an EQ and cut out about 1700Hz. Do you want this to happen to your beautiful music? Of course not.
So this means basses, very high strings, and an avoidance of the middle registers of horns or pianos (especially).
The dialog is your melody. Remember that.
If you're scoring a scene without dialog, say a chase scene, you have to deal with how the sound effects will or might collide with your music.
Love scenes are usually 100% the domain of the composer. Knock yourself out. Stay away from the 80's porno saxophone though, nobody likes the porno sax.