My method to make someone compelling is usually to program them with two big character traits that don't seem to work together (usually at random) and then rationalize how and why the person got that way, and how they embody both sides of whatever weird data points I've assigned them (Sometimes this leads to really deep emotional tension in the character - my devout Jain who's also a materialistic pop-culture obsessed fashionista. Or it could just give you somebody who's normal but oddly quirky - I have a character from North England who sings in a country-western band on the weekends - which she's a pretty normal person, but it was fun to work out the "how did that happen?"). Also, assuming it's a reality show premise, you probably should prioritize the cast in order of importance and develop the important ones farther. No "Survivor" fan ever knows much about the first two people who get voted off - but we know a lot more about the people who make it to the end. If some of those 13 characters are "cannon fodder", they can be rough sketches.
Know their purpose and make them flawed. That creates a character with depth. And people identify with characters like that. Each character MUST have something to lose. Something as got to be at stake for them. And they have to fight tooth and nail to get what they want. If they have a solid struggle . . . a deep need and their flawed personalities get in the way of their own goals, the complexities end up making them a well-rounded character.
