With some authors, the plot line of a story comes first. What's going to happen? What is at stake? How will it all be resolved?
With me, characters come first. Something happens to someone. Someone has a great deal at stake. Someone must fight to get what they want.
Who is that someone?
For me, the "someone" doing the "something" matters a lot more than whatever the "something" may be. I have to like the characters in the story or, at the very least, find them interesting. There are many ways to kill my interest in a character.
I hate whiny characters, in real life and in fiction. Whining is a habit, not an endearing character trait. There is a reason why I trained my children out of this habit at a young age--I didn't want to hear it. Nobody likes to listen to a whiner, so if a character has this unpleasant habit, the writer better have a good reason for it. And never let that character be the main character! The main character may have reason to complain, but he or she should be more focused on getting things done rather than bewailing the way things are.
I also dislike characters who are in complete control of everything. Their life, their jobs, their emotions, even their hair, is always perfectly in order and runs smoothly. Now before everyone jumps on me and says, "Hey, what about...?" Yes, I know, it seems that my character, Sheriff Rick Sutton, has it all together, BUT... yes, if you've read "At the Crossroad", you know it's all a facade to hide a lot of pain. He's not a robot; he's human and he's trying to protect himself. A lot of people can relate to that.
It's easy to understand why boring characters can be unlikable. We all have met someone who goes on and on and on about everything and nothing at all. A boring person is one who focuses almost exclusively on him or herself. While it's necessary to know things about a character, focusing solely on that one character--especially a character focusing on himself--and nothing else, makes for a story that is very easy to put down and walk away from... especially since most of us who come into contact with boring people feel trapped and unable to walk away.
I really dislike villains who have no reason for being a villain other than... well, the hero needs someone to fight. All villains, like heroes, have a goal and it has to mean enough to them to make it worth their time and effort to achieve it. True, it's very likely not a good goal--after all, they are the villains--but it has to mean something to them, something important.
What creates an interesting character is that they remind us of real people, of ourselves, even. Real people have real reasons for the way they act. We can't always delve deeply into the lives of the people we interact with in the real world, but a writer can and should do that with the characters in a story. A character with a motivation rings true, is relatable, even if not likable, and makes a story more interesting.