Sometimes it's a scenario or a character or a setting. Or just the style that they used. But yeah, never rewriting or extending their story.I'm inspired all the time by stories that I read, but not to build on the specific universe they've created. I take basic ideas and rework them into my own story. I think it's better to resist the impulse to tweak another person's story and concentrate on your own stories, albeit sometimes inspired by or influenced by others. It's better for your own creativity and artistic integrity and it's more courteous to your fellow authors.