How does NetFlix work? If they stream the content like cable On-Demand, it may never download the entire movie. I know the Apple TV downloads the whole movie and stores it for the duration of the rental period (which starts as soon as you play but not when you rent). Depending on the link speed, it take Apple TV 30 seconds to a few minutes before it says the movie is ready to watch. Although rare, sometimes it stops half way through the movie and has to buffer more content. When this happens, something goes wrong on the back-end servers as I have tested my HSI connection and it's fine. Generally I have to do something else for a few hours while waiting for it to finish downloading. (I can wait for 10 minutes and get a couple minutes of movie but this is even more frustrating).
As I said, this is rare but I've had it happen in the middle of action scenes and it makes me want to walk over to the Apple TV device, yank out all the wires and throw it into the fireplace. (One time I was really close but I then my brain pressed pause because there might be something in it that explodes resulting in hot coal burns in my carpet!)
On my device at least, Netflix buffers about a minute in advance and nothing behind. I have always wished for an alternative where I could tell it in the morning what I wanted to watch and it would download it at its leisure during the day for certain, uninterrupted viewing when it is done. Besides letting me know before I start that I'll see the whole movie, this would make it feasible to skip forward and backward as I watch. As it is, I use DVDs by mail when I can because that has those characteristics.