Di really knows her stuff when it comes to defining the nitty gritty of character.
The way to getting to really know the in's, outs, strengths and weaknesses of your character is to put them through the mental ringer a few times over. That is, take them out of the context of the story - out them under pressure, gague their reactions to a vast milieu of social situations and predicaments. Will they cave when push comes to shove, or will they persist? What is the life-affirming message you wish to embed in your narrative... or is it even life-affirming?
You could even write a biography.
Di reccommended a good source of internal monologue/reflection, a book entitled 'You'll never eat lunch in this town again'. No better or more compelling way to get inside a persons' head than through a memoir.
Basically, you are not going to make progress with these characters if you aren't willing to get confrontational. This is often more or a challenge for the writer than the character in question. The key to writing colourful, evolved fiction is to dress up every situation to the nines, and work out exactly where their quirks of personality lie.