Before answering the question, if you're interested in composing music for film, here's a very interesting article entitled:
As a provider of music for film, gaming, whatever, you have to thank your lucky stars to come upon a producer/director that lets you do what you feel is right for a project. I have been on both sides of the coin, but, thankfully, mostly on the side where the final decision was to go with what I thought was best from the start.
The best situation is where you are given ideas of where the producer/director would like to see things go -- without specifying how to get there. I'm lucky enough at the present time to be working with these kind of people.
And now, for my answer to the question posed in the title of this blog entry: Arrangers and Orchestrators. These are far and above the most screwed musicians in the history of the industry. As you see from Mr. Lee's article, they have been left out of credits since the beginning. Yet they can take a mediocre song and turn it into a hit.
What pushed me to write about this (I've thought it for decades) was listening to music of the 50's and 60's that bridged the gap between the rock/blues and and 40's pop music.
It's only fairly recently that we are finding out who the arrangers/orchestrators were for much of this music. It's about time!
For an example of a songwriter who also was an arranger, listen to Morning Girl by Neon Philharmonic. It was written and arranged by Tupper Saussy (his true name). Notice that early stereo mix! Harpsichord Rock on one side, lush orchestration on the other side. Make sure to listen through to the end -- the strings go where rock and roll instrumentation just wouldn't cut it.
How many songs are there that were made by a musician's invention of a musical hook? The instrumental riffs that bring a whole song to mind, like the riff in My Girl. The guitarist is Robert White.
By the way, you did notice the orchestral arrangement in the song, didn't you? That's the Detroit Symphony Orchestra -- they were involved in many Motown recordings.
These days, there is no excuse to leave out credit for any of the people who bring a song to life.