This has been discussed frequently and there's been some disagreement. Some people say you cannot view your iTunes content with home sharing unless you have an internet connection. Others claim that you can.
All I know is my own experience. When my internet connection goes down, I can't view the iTunes movies on my Mac Mini. Although I couldn't watch movies on either of my two AppleTVs, I was able to watch them on my MacBook Air with home sharing while the internet connection was down. So this seems to be an issue with the AppleTV and not home sharing in general.