Flex TV. is a web application that interfaces between google Assistant and Plex to allow you to control playback of media using Actions for Google or IFTTT.
Additionally, it interfaces with the API's for Sonarr, Radarr, Couchpotato, Sickbeard, and Sickrage.
It also has customizeable webhooks, so you can basically control anything that has an open API with your voice. If you can send data to a URL to do a thing, you can program it in Flex TV.
Interested? Check out the Wiki to get started.
Also, there is now a web-only version in development, currently supporting only non-cast devices. I will be looking for beta testers for this service in the coming weeks, so if you're interested, please send me a PM with the email address you are registered to Plex with, as well as a hangouts address I can contact you at with further details. Read all about it here.