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: 
I think one of the problems with character development in novels is the author's liberty to spend periods of time explaining a character's motivation to a reader without the character actually having to say or do anything. A chapter that has the ostensible position in time and space of someone sitting in a chair, reflecting on a dramatic problem, is something that crops up from time to time. That reflective period can info dump a whole heap of incidents in summary and leave the character in a position where the next things they say and do are obvious to everyone.

The problem with that is that doing this is dry and it restricts the things that you need to have the character do in order to establish character through action. It also means that you miss opportunities to flesh out character through interaction and action.

There is nothing wrong with having a character make a seemingly bizarre action that they later have to explain, possibly to another character. Also it is possible that characters don't always agree with each other's motivations. These nuances of character are what flesh out the interactions and make each character grow and deepen.

I lucked into character development boot camp, I did a film-making course where I wrote a lot of scripts. They weren't great scripts but they taught me the nuts and bolts of what do when you can't just gear-shift into "exposition mode" and have a slow beat of characters reflecting on their own pasts.

If a story can get by with out that kind of thing it will be richer for it. It is a path that should only be taken when absolutely necessary. Always explain exactly as little to the reader as you need to in the moment to keep them reading the next few paragraphs. Info dumping is the gentle story killer, smothering under the guise of explaining.
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.