They were already part of the story, I'm just fleshing them out in the flashback. But the trick is to just let the main storyline sit idle while you build the characters, if you try and tell the story and build the characters, something always suffers, there's only so much you can pack into a sentence; also I found that if I try and advance the story and develope the characters, readers tend to ignore the character developement and focus on the story, so I seperate the two, give them their own chapters.