You're right, I was at one time working on a characterization course. Characterization is my favorite of the major elements. I actually devised a really intuitive system for creating unique, captivating characters every time. Only my system works big to little. Where most people start on the particulars (appearance, job, likes and dislikes, etc.), I prefer to start with other priorities -- purpose and personality. Because none of that other shit matters if your character doesn't have a purpose, a goal. What are they trying to do? Preventing them from getting to their goal is what creates conflict and tension (plot). And a great way to do this is to make their personality quirks and decisions backfire on them, so it's essentially their own fault. At any rate, I call it the P3 Method, and I'm thinking of writing a series of tutorials on it instead of offering the course.
The method works big to small. When creating your characters, I recommend starting with their purpose first, then doing their personalities, then doing the particulars last. Lots of other people only do the minute details. Those are superficial. Surface-level. Cosmetic. In doing the bigger items first, you can develop a multi-faceted character that stands out from everyone else. What is the purpose in the story? Number one priority. Figure out the goal. Then figure out the personality. Not what they like or what they do. These things don't define who we are as people. But what makes us interesting. Actions and mannerisms. I like to figure out some positive traits and some negative traits. For example, some people are really afraid of abandonment. Why might they be like that? Make something that happened. Determine a list of experiences and stimuli that contributed to who they are on an emotional level. What pisses them off? What makes them happy?
Lastly, I figure out the particulars. What do they look like? What are their skills? What other knowledge do they possess? What do they talk like? Other superficial details. Put these all together and you have a unique person. Do this for all your characters and because you'll know their personalities, you'll be in a better position to have them interact with each other and respond more in accordance with who they are. They won't be cardboard. They'll argue. They'll fight for what they believe in. They'll have conflicting views on things. They'll see the world differently, they'll act differently. Figure out the 3 Ps and the heavy lifting is done for you.