Billable Hours

As the season here winds to a close, the production manager sees I need more to do. She invites me to work on other freelance projects while on the clock.
While on-site, I bill hours to the client. What to do, if I'm expected to stick around but have no work to do?
This has happened before. While freelance proofreading here for a week, I filled down time by reading this book.
Corporate proofreading gigs can't be all the same. My first freelance proofreading gig here allowed little downtime. I guess retail has stricter production schedules.
Anyway, I'll focus when the job hits the desk. Let me say it takes effort to combat the billing-and-hours-equity paranoia, but on-site, I bill. Clients, the flat rate, it's an advantage.
As always, Latent Print welcomes your comments.