I think all writers are always collecting characters as we go along. Not just characters of course, we're collecting EVERYTHING. Bits and pieces of story. An interesting dynamic between people. A theme. A great character back story. A cool occupation. The look of someone's eyes. A burning ambition. Hundreds of thousands of bits of flotsam and jetsam that we stick in the back of our minds like the shelves full of buttons and ribbons and fabrics and threads and beads in a costumer's shop.