If you’re writing a novel, you might want to have the whole thing planned out in advance before you start to write it. But I don’t do it that way.
Ask yourself this: Are you a “minor character”? Your book will be full of them. Maybe it’s someone who comes onstage for just a moment to say “Here are the gum boots that you ordered, madam,” and then exits, never to be heard from again. It’s a minor character, and you don’t even need to provide him with a name.
But he has one. He has a life. In his own way, which may never show up in your novel, he has importance.
And if it turns out that you’ll need him again–well, there he is.
This happens a lot for me, in my books. A character has a walk-on, but it turns out to be much more than that: he may even develop into a major character. Orth started out as just a henchman of Lord Reesh; but now he’s Lord Orth, the First Prester. Duke Esdras, confined to a wheelchair, will produce the climax of my current book, Ozias, Prince in Peril. I needed someone to do that, and there he was. Most of your minor characters will remain minor–but you never know. Don’t be too quick to dismiss them!
[And yes, I still have no access to my stats page, no idea of how many views I’ve got today, and heaping piles of frustration.]