If you're casting from an Android phone or tablet device you can activate subtitles by following the steps below:

1. Make sure you're in the Tubi app and that you're connected to Chromecast.

2. Tap on the Chromecast controls at the bottom of the screen:

3. Tap on the "CC" button to the right of the play button:

4. Tap on "English Subtitle" and press OK.

Now subtitles will be activated on your big screen!