Character, I think, is the single most important thing in fiction. You might read a book once for its interesting plot—but not twice.
— Diana Gabaldon
Character development is simple enough to define and infinitely complex to deliver. Who is this person? What do they look like, sound like, how do they think, what’s their background, what are their hopes and dreams? As fanfic writers we have some of that provided for us, but it’s important to keep even canon characters’ known traits in mind as you’re building your story around them. For instance, even in the most alternate of alternate universes, there’s no way Daryl Dixon is going to be the public face of some enterprise--he’d be the guy behind the scenes, watching over everyone else and making sure they’re safe and everything runs smoothly. That’s his character. If you diverge significantly from that, you'd better have an explanation for it that makes sense in the context of both the story and the character's backstory.