Who are these characters and what do they want?

I suspect creating a character is usually more of an organic process than a construction project. It's possible to 'build' a character from standard parts - as in the less sophisticated type of role playing game - and then put them in a setting and see what they do. But I suspect most strong fictional characters grow out of an idea of what the character wants, what their problem is, and the rest - their job, physical description, personal attributes etc - come after that. The want or problem being what makes them interesting enough to hold the reader's interest until it gets resolved.

Which would be why it's so much easier to start writing a story than it is to end it - it's easier to identify wants and problems (fictional or real!) than to resolve them.