I'm noticing a lot of similarities between painting and writing.
First of all, you need to know your materials, like watercolors, and supports, like paper, which translate to vocabulary and grammar. Sure, there are the singular geniuses that create brilliant works without a single lesson, but I think those people just know the craft intuitively. Most of us have to learn it through rote until we can feel it that way.
An artist learns that highlight and shadow gives depth and that you can't give everything the same attention or the eye won't move through the image. That was my big mistake in my first novel and I am seeing it in other new writers. You can't write eveybody's story at the same time. If everything is clamoring for the attention of the audience, nothing stands out.
What is the point of the story? And for me, a lot of paintings are stories.
It is important for the writer to know his characters. Even the minor ones. But some of that information needs to stay in the notebook and not go into the story.
When the detective takes a cab to the crime scene we don't need the cab driver's life story unless it contributes to the story. Maybe he was the last one to see the victim alive, maybe he saw the killer. Paint him him in lightly and let the eye wander back from the boldly drawn detective to the driver. Let the audience wonder - what part does he play?
However, if he doesn't play a part and the audience is left wondering about him, the story will be unsatisfactory. Maybe you rendered him so well, that you can't bear to lose him. You know his family, the sick mother and child with a handicap, the aging relatives he visits on the weekend. Either give him a place in the story or let him go. Save all that luscious detail for another story. Because it's dimming the main character's spotlight, (oops, just stumbled into a theatre metaphor, but I think you know what I mean.)
God forbid all your characters are so well drawn. How will the audience know which one to care about?
Being a good artist is all about learning to see in color and shape and negative space. The writer needs to think in plot and pace and overall arcs.