I was always under the impression that the composer wrote the piece, and the "arranger" was the one who made it "fuller" by arranging additional instruments for the piece, adding chords behind the basic melody, etc, but I wanted to know for sure and get a definitive answer.
I was wondering this after watching the credits and hearing the theme from the film Unforgiven Listen here
In the credits it says the score was written by Clint Eastwood and arranged by Lennie Niehaus. I imagine Clint Eastwood was plucking a guitar and came up with the simple melody, and then Lennie Niehaus arranged (composed?) the final composition with accompanying strings, percussion and backing chords.
Three questions I'm hoping to have answered in one reply:
1. Is Lennie Niehaus considered both the arranger and composer?
2. Is this a common way for scores/classical compositions to be written? With someone who is very good at writing basic melodies writing the melody, and a professional musician (usually with a degree in music composition I assume) composing the piece for a full orchestra?
3. If yes to the last question, can one be considered a great musician if he or she simply writes the melody and has other more learned musicians "fine tune" and orchestrate the piece?
For example (in regards to number three), Franz Liszt was an incredible virtuoso pianist and could arrange pieces brilliantly, but as for his completely original compositions, I do not know of any being considered that "great" (The Hungarian Rhapsodies were almost entirely based off gypsy folk tunes). He just (re)arranged the original melody.