I was thinking that as well as character building, there's also character dynamics (the way they relate to each other) and you could ask yourself questions that would involve all of them . For example, I was reading this TErry Pratchett interview where they gave him some of his main characters who all like to be in control and asked him if they were all having tea, who would pour? I'm sure there are better, deeper questions than that, but I thought that might help. I'll try to think of some if you want.