Locking chapters on AO3 is something I figured out after a bit of trial and error, and honestly, it's a handy feature if you want to control who can access your work. When you're editing or posting a fic, there's an option under the 'Privacy' section where you can restrict access to registered users only. It's not chapter-specific, though—it applies to the entire work. If you're looking to lock individual chapters, AO3 doesn't have that feature built in, which can be a bummer. I’ve seen some creators get creative by posting locked 'placeholder' chapters with notes explaining they’ll unlock them later, but it’s a workaround, not a perfect solution.
What’s cool about AO3’s system is how it balances accessibility and control. Locking a work means only logged-in users can read it, which cuts down on scrapers and keeps things within the community. It’s a trade-off, though, since some readers might miss out. I’ve debated using it for my darker fics, but I always end up leaving them open because I love the idea of anyone stumbling upon them. If AO3 ever adds per-chapter locking, I’d probably use it for explicit or intense content, but for now, it’s all or nothing.