That's a good question. Usually I have a boy in mind when I think of the idea, but if I'm not sure or can't decide, I think about which boy would work best in that role, almost as if I were casting them for the story.
Like for Curtain Call, the idea always felt like a Nick one to me. Even if I had wanted to use another boy, I don't think the idea would have worked as well for any other guy. I needed a guy who was "unattached," who could keep that kind of secret, and all the other guys were married and closer to their families than Nick. All I had to do was have Lauren break up with him to make it seem plausible.
I always had Brian in mind for Secrets of the Heart just because it seemed the most realistic, although if I had waited a few more months to write it, I could have used Nick in that role instead and still felt like there was some basis in reality. But I'm glad I ended up using Brian because he and AJ, the other main character, are so opposite that they created an interesting juxtaposition that I don't think I could have had if I'd used Nick and AJ or any other combination of Boys. Also I haven't written as many Brian stories lately, and I feel like I've written a lot of Nick the last few years!
Has anyone ever written a story about one boy and then later thought, "I wish I had written this same story about [different boy]?"