Buying Movies on Apple TV 3rd Gen

I recently purchased a film and I was wondering if you had to download it once and watch whenever or wait for it to buffer/prepare each time you wanted to watch it ? Also, I realise to download a 1080p movie it takes a while to prepare but a movie stopped loading at like a 3rd across the bar and was letting me watch it, so I left it for a bit like 30mins and the bar had not moved so I Was wondering wether it would load while I was watching it or would the movie just stop when it reached that point?