I didn't realize at first what she said, but when it hit me, I was shocked. I understood then why some of my books took very well to a follow-up whereas others never got up enough steam, and I'd left them as stand-alones. By then I'd read hundreds of novels and completed an MA in English and no one had ever said it that way: a sequel needs to be more than just another book about the same characters.
We'd discussed trilogies, of course, but trilogies divide up neatly into their own setup, intensification, and conclusion. It's different when broken up over two books. Ideally your two-book deal will have an overall arc: the first book will address the hidden need of the character, but the second book will resolve it more permanently or in a more complete form.
More to the point, a great novel has characters with specific needs who have been put together by the author with a specific purpose in mind. If the author didn't intend to have that second book on the horizon, but sits down to write one, the novel won't work if all the central needs of the characters have been resolved.
And there lies your tightrope: your novel needs to stand alone; but in order to have a sequel, the characters need to be able to take their resolution further than they did in the first book.
When you write a novel about a character, hold one thing in mind (told to me by author Alysse Aallyn at the 2010 Connecticut FictionFest): this should be the story about these characters' lives. Ten years from now, there won't be a better one. What you're telling now needs to be the moment these characters, at the end of their lives, will say it all changed: the pivot-point.
Therefore you the author owe it to them to give the entire story its own harmony and its own set of central questions, no matter how many books it spans.
If not, then what you end up with is a book with another book tacked onto the end, the second book having a different feel, different scope, and an almost superimposed set of needs that should have been established (and worked on) in the first novel.
At the end, my agent told me to make sure that closing chapter points in the direction the next book will head, while still fully wrapping up that first book. The closing chapter of the first book, she told me, sells the second book.
And if the needs, the drive, and the conflict of the second book will be that dramatic a turn from the first book, possibly a sequel is not for this story. It might be time to end the story and start a new one.
Jane Lebak is the author of The Guardian (Thomas Nelson, 1994), Seven Archangels: Annihilation (Double-Edged Publishing, 2008) and The Boys Upstairs (MuseItUp, 2010). At Seven Angels, Four Kids, One Family, she blogs about what happens when a distracted daydreamer and a gamer geek attempt to raise four children. She is represented by Roseanne Wells of the Marianne Strong Literary Agency.