This new Knowledge Graph feature uses schema.org structured markup from websites. "With the recent launch of critic reviews in the Knowledge Graph, we've leveraged this technology to once again provide publishers with an opportunity to increase the discoverability and consumption of their reviews using markup. This feature, available across mobile, tablet, and desktop, organizes publishers’ reviews into a prominent card at the top of the page," informs Google. Here's the help center article for webmasters.
You can search Google for [minions reviews], [reviews for ex machina] or ask Google: "did Fantastic Four get bad reviews?" and you'll see a list of reviews. Critic reviews are also displayed in the regular cards for movies and Google plans to expand this feature to books and TV shows.