So, what exactly can I expect from a fetal nonstress test?

I’ve checked online and I’ve seen they can last anywhere between 20 – 90 minutes. And sometimes you have to retake it? Will I be allowed to move or go to the bathroom? Should I bring a book? Can I sleep?

Am I even asking the right questions? I know what it’s suposed to monitor, and I’m not too worried about that. I just don’t know what to actually expect. Considering I’ll be doing these twice a week until I deliver, I’m a bit curious.