synching and sharing

If you are watching movies from the shared section of the menu you have linked your itunes library as a secondary library. If you change this to a primary library you will be able to sync a selection of media and stream the rest as you indicate you want to do.

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You can set custom sync settings in iTunes so that it only syncs the files you specify, and you can stream the rest if your iTunes library is online. Just make sure you don't check the option to show only synced content.

If you only have a shared library and haven't set up syncing, you will first need to run the sync setup from the ATV settings menu.