Image via WikipediaI'm an executive summary kinda gal, and as there's a huge amount written on this topic, I thought I'd distill it to its essence. Of course, a complicated idea can always be expressed simply, and that doesn't necessarily mean it's easy to do. But if you can think of things in simple terms I think it makes them feel more achievable.
So this is my cheat sheet for character writing.
You establish a character and make them live for the reader through expressing the following things:
What they FEEL. I'm not sure this one needs much explanation - other than to say that it's good to express feelings and reactions sequentially, in the order in which they would naturally occur; feeling, followed by reflex action, followed by conscious action or speech, (or a POV character's internal dialogue). More on Dwight Swain's techniques for writing about reactions here.
What they SAY. Dialogue gives a great insight into character, and what's even better, it's easy to read. Dialogue speeds up the pace of your writing, so if you make dialogue work for you, readers will be turning the pages nice and quickly, and learning about your characters and what makes them tick without even realising it. Dialogue is often the key to SHOWING your reader something, rather than TELLING.
What they DO. Sometimes actions speak louder than words. A tender touch, a flinch - sometimes that's enough to say it all.
These three things will allow the reader to understand and share your character's MOTIVATION, and also hopefully how they grow and CHANGE as your story unfolds.
Whether a story is plot-driven (and characters are mostly reacting to external influences) or character-driven (where a character's choices directly influence how the story unfolds) the above still stands. I think.
As for making your characters compelling? You know as well as I do that there's no easy answer to this one.
Write from your heart. Your mind.
Be guided by your creative impulses. Your life experiences. Mad whimsy.
And then hope against hope that your reader shares the vision you've poured onto the page.