If patients are doing a great job sitting still for a lengthy procedure, let them know with a compliment. They may have no idea whether they're being a good patient or not. They'll appreciate it and feel better about the appointment.
Do be supportive. Saying something as simple as, "You seem very relaxed in the chair! You're a great patient. Did you know that?" will instantly put patients at ease, especially if they aren't quite as relaxed in the chair as you say they are!
Don't assume they're OK. If they aren't OK, there's no guarantee that patients will tell you. Checking in with them periodically keeps them focused on the positive outcome that they will experience.