Pizik's made the point that all literature has a theme. Sometimes you're beaten over the head with the theme (think Brave New World and 1984) and sometimes the theme is hidden behind the curtain and you don't really notice it.

The plot is what keeps you reading, but the theme is why you remember the book later. It's what grabs you and holds you.

Theme either comes first. "I'll write a novel about how a strong character can rule his own destiny despite society's expectations", or it can come later, "Hey, my main character does his own thing regardless of what people want him to do".

If you start with a theme, you run the risk of overdoing it. If you find your theme later, you run the risk of muddying the message with scenes that don't advance the theme, or you need to do massive reworks to make the theme work.

You can have multiple themes, but there should be a primary theme. Having one theme for a main character and a related theme for a minor character is a strong way to present multiple themes.

He punctuated his talk with exercises and surveys like "think of your three favorite books. Now think of their theme. Can you state it in one or two sentences."

One or two sentences... That's the elevator speech. Next time someone asks you what you're writing, don't try to explain the plot, subplot, characters and setting, tell them the theme. It's a book about good triumphing over evil, or how simple acts affect larger events, or how love will overlook failure, or something.

If you pay attention to your theme, your characters will seem more real and wil behave more consistently.