If you're like me and you subscribe to tons of different services, you'll want the Movies Anywhere application for your Chromecast.
This app integrates with Google Play Movies, Netflix, Amazon, iTunes, Disney Anywhere and more and combines all of the movies available to you into one place. Yes, it's amazing, although it doesn't always work 100%. It's basically your entire digital library in one place. This includes those "digital copies" of movies you buy at the store too. Once you have it ready, cast any movie you own to your Chromecast devices.