Question: I'm 1/4 through my first novel, with a pretty solid 12 page outline of the whole thing. After years of reading books about story structure, I think I have a good grasp on how to make a plot work.

My characters, however, are turning out to be quite flat, uninteresting and generic. They do have motivations and I'm putting a lot of effort into making sure they act accordingly, in a logical way according to their goals and the story seen from their perspective, etc. But they don't feel "alive".

I've read about the archetypes in The Writer's Journey, but that's about it. How can I make my characters come to life?

Answer: 
Perhaps the problem you are having is you are thinking about it as "character development" not storytelling. Many writers start off working on their work with a formulaic engine of what is going to happen. They know where, when, who and why. They believe this will make their story compelling since they dot every "i" and cross every "t".

* If your characters move through your story as if they already know what is going to happen and are simply reacting to it, this removes the sense of tension you are looking for and thus they cannot "develop" or change.
* Yes, there are characters who are not changed as they move through the story. But in those stories, the environment changes completely around them. This usually means in the end of the story there is no place in the new world order for them. Think Rhett Butler in Gone with the Wind.
* Character development is what happens after something unexpected happens. If your characters are not experiencing the new, the different the unexpected and are not changing because of it, they are not developing and will remain flat as their responses to events is unchanging.
* It isn't enough to differentiate them by style of clothing or behaviors you write in with their descriptions. You have to find a way to make the reader feel them, want to know them, fear them, fear for them.
* This can only happen when you are willing to present them as fully as possible, revealing them the same way you would find if you peeled an onion. Yes, there is more onion beneath the first, but your goal is to describe the deeper layers of your characters. Just like with real people, what's on the surface is only the tiniest bit of their character. Their true desires, fears, ambitions, may stay completely hidden due to forces outside of their control, i.e. their religion, their family expectations, their social obligations, the nature of their desires, which may be considered potentially perverse or taboo in their society but not in others.

This is the tension you seek when you are looking for character development, the push-pull of the character toward his/her goal while dealing with opposing forces, inner motivations and the overall plot of the story which is what moves the character into opposition with his environment in the first place.
You need to consider your characters outside the plot as well. They'll have likes and dislikes that have nothing to do with the story, funny mannerisms they're unaware of, things that set their teeth on edge, irrational fears. The strangest things will get them into certain moods.

Let's take a real person, Charlie (name changed to protect the not-so-innocent), as an example. Charlie hates M&Ms - they make him sick, after he ate too many before a long car ride and produced a bag full of multicoloured puke. He's terrified of worms because when you chop them in half, they still wriggle about - but spiders, cockroaches, and other bugs don't really bother him. He gets bloated after big meals and can't stop burping. He can't stand it when people sit on his bed with pants they've worn outside of the house. He hates afternoons because they put him in a lethargic, melancholy mood. 

Were he in a story, it's likely none of this would be directly related to the main plot or his motivations. But it's the little things that people say or do or feel that bring them to life. And when you get them down, you'll see them appearing in your story quite naturally. For example, Charlie may be having some friends over. He may turn down the M&Ms they've brought, and keep them out of the bedroom so no one sits on his bed.

It may also be useful to do some online personality tests, as they'll get you thinking about how a character would react in various situations, and how they'd respond to some tricky questions.