DM's reply is also valid, though.

For my money, it's entirely legitimate to develop a character as you go along, figuring out exactly who they are as they react to the events of the story. There's every chance you're not going to end up with a compelling story that way, if you're writing without planning, but DM acknowledges and is aware of that.

The key, as far as character development goes, is consistency. Once you figure out who your character is through this process, that character should remain consistent and true to what you've discovered about him/her.

This is where a lot of inexperienced writers run into trouble; their characters will act one way in one situation and a different way in another situation, without any logical or explained reason. Without consistent characterization, and without logical reasons for deviations from that characterization (which in themselves would be simply another facet of the character's identity), the reader will often have a hard time feeling as though the character is a real person with whom they ought to empathize. And some inexperienced writers, when faced with the notion that their characters aren't consistent, will fall back on the "it's just my style" defense, or "art doesn't have to be just one way", or something similar.

Which is fine as far as it goes, but if a person wants to improve they have to understand that style is not an inherent, inborn part of a person's artistic makeup, like something from genetics. Styles change over time, they improve or degrade, they shift to match the needs of the story, the genre, the audience, etc. There are plenty of good reasons to use a style or a storytelling method that's unconventional, but if the end result is not good, claiming that "it's just my style" does not magically make it better, which I believe is what BG was getting at. A writer has to be able to find a style and a method that works for them, but they also have to be willing to attempt to improve on those skills where needed.

But none of that is a criticism of DM. If his method of discovering character leads to consistent characterization, then all's well. He's clearly aware that it doesn't always lead to a good story. But I guess the point of this response is that I don't think BG was trying to put him down just for the sake of doing so, either; lack of consistency due to seat-of-the-pants storytelling styles is definitely a problem for a lot of writers, and it's worth pointing out. Lord knows I used to suffer it at times.