I sent off a draft last week after applying a few more revision suggestions. Two of the suggestions were basically logistical issues within the story, but the third suggestion was something a little bigger. My main character's interests were clear, and so were his reactions to the people around him, but his own personality was harder to define. Why couldn't he be more like my secondary characters?
My agent was right, and I did some work to flesh out traits that previous drafts had hinted at but not emphasized; I hope I've resolved the issue. But I also did some mulling over of why I had this problem in the first place. I definitely consider myself more of a character person than a plot person, but somehow, I'd ended up with strong secondary characters and a protagonist who mostly just reacted to them. This hasn't been the case in all my writing projects, but it also isn't the first time it's happened.
I typically write in either first or close third person, so both the reader and I go through the story, moment by moment, right with the main character. As in life, other people happen to us. As in life, we see others from at least a bit of a distance. We know that not every single moment of our own lives is character-defining--we might be mild-mannered but occasionally get upset about something that happens to us; we might be early risers but occasionally sleep in because we've had a tiring week. We know that we react to things that happen in whatever way makes the most sense--right?? So the things that happen are more important to the story.
Now that I realize this is a trap I can fall into, I'll react to that information by spending more planning time on my protagonists. It seemed obvious that I needed to do this for other characters, especially antagonists; after all, readers need an explanation for why they do the ridiculous things they do. I guess that's true of the story's hero(ine), too.