Watch now currently includes Google Play, but if you click on it for a movie you get to a page that says “Purchasing movies is no longer available on Google Play”. It looks like the id query parameter on that page matches the id on YouTube where you can rent it, so it would be nice if you could link to the movie on YouTube movies instead.
E.g. instead of linking to https://play.google.com/store/movies/details/Collateral?gl=NO&hl=en&id=Cwx_TbKiZIM link to https://www.youtube.com/watch?v=Cwx_TbKiZIM