A couple of editors have commented that some secondary characters in my YA ms aren't "developed enough." I'm guessing they meant the characters were cardboard cutouts. I tried to address this problem by showing more of these characters' motivations, gave them more to do than just be a sounding board or antagonist for the heroine, etc, but I'm not sure I've gone far enough. At the same time, I don't want the secondaries to take over the whole show. Could you please talk about character development, what works for you and what doesn't?
The trick seems to lie in giving your secondary characters personalities, histories, and priorities of their own, without regard to your main characters. Then picture the characters interacting. (Not everything will go as planned, and that's what makes writing life-like.) Give your secondary characters a small chance somewhere to show us their point of view—even if it isn't one anyone else in the book agrees with. Give each character a chance to petition for the readers' sympathy—even if they don't get it. Give each of your characters a sense of self, and they'll become the currents your main character swims in and against, rather than simple scenery.
What is your opinion of writers submitting to agents and editors at the same time, especially for picture books? Say a ms was well received during a First Pages session and the editors said they would like to see the rest of the book, would the writer be better off securing an agent before submitting or should he go ahead and submit on his own?
The decision to find an agent or not is a personal one. There's nothing wrong with submitting to editors and agents at the same time; go about this however suits you best.
EA, you may have answered this, but I was wondering about the rejections you give to agents, are they more honest than the ones to authors? Before an editor said yes to my manuscript, several said a variety of things like what you've described here, "strong writing, character, plot, but not right for me." There was also at least one, the writing isn't strong enough. I'm curious if you work harder to be clear about your rejection with an agent, because you are trying to educate the agent as to what you are looking for, and developing a relationship, or if the bottom line is you don't have much time to spend on manuscripts you don't want because you've got to find and work on the ones you do?
As with authors, I'm more likely to be specific if I think there may be some eventual reward for me. Very promising authors and agents who I know have great taste get more feedback for the reasons you mention, as well as to encourage them to keep trying me with manuscripts.
Non-specific responses are not necessarily a sign of no respect, however; it's usually just a sign of no time. Most authors and agents don't really need encouragement to keep submitting, after all.