I am not an expert, but since this is an area I've been studying and refreshing lately, I'll chime in on character development. I own multiple books on this subject, so summarizing in a few paragraphs may not be possible, but at least this can point you to other areas for additional research.
I think the most important thing about character development is a solid understanding that your character has a past, and that everything in that past has an affect on how they act during the course of your story. The writer must understand the major events from birth to story start, including other people in the character's life. Where were they born? What level of education? Hobbies? Interests? Broken home or parents still together? Siblings? Oldest, youngest, middle child? Dropped on their head as a child? Mental or physical disorders, learning disabilities, a facial tick? All of these details can help add depth to your character even if not all of them end up on the page or screen. Although your character may be a work of complete fiction, they must be REAL to you, an organic, living being (or inorganic in the case of Wall-E, but it's the human characteristics a writer adds that help us empathize with even non-humans).
Second most important aspect is how your character grows and changes throughout your story. This is most often referred to as the "character arc". Characters should grow and change, challenge their traditional belief systems, or have a solid reason as to why they don't, won't or can't change (for example, the woman trying to convert a homosexual man or vice versa). There are several methods for changing your character, the most extreme of which is metamorphosis where your character literally becomes something else (e.g. Cat People).
The third most important aspect is the "character web" where other characters interact with your main character to help reveal these small details and facilitate the process of change. What better way to demonstrate past sibling rivalry than to throw an older or younger sibling into the fray and have them dig up old wounds. If this is the focus of your story, perhaps a sub-plot arc would be for them to finally heal those wounds during the story. Co-workers, bosses, family, friends, perhaps even pets, when strategically placed and similarly developed, often contrasting with your main character's belief systems, are key to bringing out and building depth to your main character.
Study archetypes to get an idea of how basic character traits have existed and worked in story for thousands of years. "The Hero’s Journey" is required reading for all aspiring writers, in my opinion. Also study the archetypes as defined by Carl Jung. His book, "Personality Types", should also be on your shelf.
EDIT: Other recommended reading:
"The Writers Journey: Mythic Structure for Writers" - Christopher Vogler
"The Anatomy of Story: 22 Steps to Becoming a Master Storyteller" - John Truby
"Creating Unforgettable Characters" - Linda Segar
"Writing Great Characters: The Psychology of Character Development in Screenplays " - Michael Halperin
"Save The Cat! The Last Book on Screenwriting You'll Ever Need" - Blake Snyder
"The Power of the Dark Side: Creating Great Villains, Dangerous Situations, & Dramatic Conflict" - Pamela Jaye Smith