Writer Nicola Morgan has some compiled an invaluable non-exhaustive list of questions to ask your characters:
What is your worst fear? And your second worst? (Likely to be part of the conflict and tension.)
What would you most like people to know about you? (Make sure it’s obvious, then.)
What would you most like to hide? (Every hero has a flaw.)
What would you most like to change about your life? (Could be part of the conflict and motivation; could be sub-plot.)
Why should we care about you? (Because if we don’t, we won’t read on.)
What were you doing before this story started? (This informs your back-story.)
Do people understand you? If not, what do they get wrong? (Makes your character more real because it informs interaction with other characters.)
If I met you for the first time, would I immediately know what you were like or would it take a while to get to know you? (As above.)
What sort of people like you? Do adults like you? Do boys like you? Do girls like you? Why? Or why not? (Helps place your character within the real world instead of just on the page. It may also inspire some ideas for painting your character richly but subtly.)
Are you happy on your own? (As above.)
What are you going to achieve in my story? (Crucial for plot, since character drives action.)
What trivial but annoying habit do you have? (Makes character more real. Character can show this habit when angry / sad / stressed – helps you show without telling emotion too much.))
What trivial but annoying habits do you dislike in other people? (As above.)
What four (or three or five) adjectives best sum you up? (Helps you remember traits to paint most strongly.)
Are you going to die in this story?** Should you? (Informs plot and interacts with reader’s engagement.)
And on her blog, writer Nik Perring chips in too:
It is really, really, really hard work. And exhausting. I mean, writing the thing’s difficult enough (and that’s after all that time spent learning how to write well, after all those stories we’ve given up on) and then the submitting, the editing. But once you’ve signed that contract it’s as though, to a point, you’re starting from the beginning again. You have to work hard to promote your book. Your publisher will do what they can but, really, the hard work’s down to you.
Don’t expect any favours. From friends or from reviewers. Of course some are lovely and only too pleased to have a look at your book and tell their readers what they think of it … but I’ve heard from people I’d not heard from in years and years and, in contrast, some of the people I’d have thought would have been the most pleased for me have shown little or no interest at all. And, I suppose, why should they? As a writer, published or none, you’re not owed anything.
Be hopeful but be self-critical. It’s a high standard you have to reach and make no mistake, you ARE competing with the best in the business. And what makes it harder is that they’re known – by readers who buy their books and by publishers who know they’ll sell the books. But they were unpublished writers too once, you know! And they got to be where they are now by working very hard and by not giving up. And probably, by trying and failing a few times too. Remember: nothing’s lost.
My last piece of advice though, is this: enjoy your writing. It won’t be fun all the time, but you should do it because you enjoy it.