As part of my teaching of aspiring authors, I am often asked what they need to consider when writing a novel. I always say that, if you are thinking of writing a novel, start by asking some key questions:
Why do you want to write it - does the story sustain a novel?
Can you really do it? This is a long slog.
Can you sell it? Has it been done before? Is your idea a new one or are you able to re-tell an old story in a new and fresh way?
Who will tell your story? Third person - can see everything all the times - or first person - allows for a certain informality but restricted to what happens around them?
If they decide to go ahead, my advice is always:
Write a detailed synopsis first: map out your story. I do it chapter by chapter. I very rarely stick to it but it gets my mind focused. I find it also helps to be able to summarise a novel in a single line.
You need to develop various plotlines to sustain a novel. Novels can work with one single story but the best ones tend to have sub-plots involving other characters
You need a strong sense of place and strong characters, which goes back to my belief that there are three points to the writing triangle - story, sense of place, sense of people. Get them right and the rest flows from it
As the story develops, there has to be a pace, a sense of things happening, so the reader does not have chance to become bored.