Well, frankly, that’s ridiculous. My characters always have a huge backstory, that I might never put in the book, because I am not writing a 500 page biography, but a snapshot of a moment in their life.
I’ve learnt, from my favourite authors, that even characters that have one or two lines should have some sort of backstory. I may not mention why they were there, or what they look like, or what their secret sorrow is, but I know, and I like to think it gives my minor characters life.
We, as authors, may not ever mention that a certain character is gay, or another was bullied as a child, but if we know that, if we know their lives, it somehow filters through, I think. The readers can tell that this character has a life outside of the few pages we see them.
The characters live on in an authors head. Just because it didn’t make it onto the final page, doesn’t mean it didn’t happen.