there are modern artists who splater paint on a canvas and make millions and there artists like banksy who has painted so much and inspirated so many and hasn't made a dime. they are all good artists whatever they paint. it might be better for an artist to learn the principles of design first because that will probably appeal to more people (to have realistic art) but understanding the principles doesn't make you great. some people are just born with it.
It can mean a composition in two or more movements, or separate sections, played by a small group of instruments, having no more than three independent parts.
I think that Captain Renault didn't arrest Rick because he trusts Rick and what Rick is doing. He thinks Rick is a smart person and that his judgement is good. I also think he didn't arrest Rick because he is loyal. The film shows many different competing and conflicting motivations in the characters.