Taken from Google's Chromcast support page.
Second section, Bullet 3. What do they mean by this? I thought Chromecast was platform agnostic?
Using Chromecast, you can cast movies & TV shows from the Google Play Movies & TV app from your Android phone or tablet to your TV.
To Cast from the Google Play Movies & TV app to your TV:
- From the Google Play Movies & TV app on your Android phone or tablet, select the movie or TV show you want to watch.
- Press the Cast button in the upper right corner of your video and select your Chromecast from the list of available devices.
- You will see a notification on your TV that your device is connecting to your Chromecast and the movie or TV show will start to play on your TV.
You may continue to use your Android phone or tablet to perform other tasks while your video is playing. To control your video, select the Cast button from the notification bar at the top of your device. All video functions, such as Play, Pause, Rewind, etc, can be controlled from your phone or tablet as they normally would. To disconnect your device from your Chromecast, select the Cast button and select your phone or tablet.
- Video quality may appear poor for the first few minutes. This is expected and will improve as you watch.
- You may cast movies and TV shows from the Play Movies & TV app that you have rented or purchased from Google Play. At this time, you cannot cast rentals that have been downloaded to your device.
- Casting from the Google Play Movies & TV app is not currently available on iOS iPhones, iPads or from your laptop computer.
Note: Chromecast requires Google Play Movies & TV app version 2.6.6 or higher on Android. To update to the latest version, please visit Google Play.