In a recent interview, singer Ed Sheeran shared a story about Kendrick Lamar and Eminem that he had heard secondhand via Rick Rubin. Apparently, the first time Kendrick and Em worked together, for "Love Game" off Eminem's Marshall Mathers LP 2, Eminem checked Kendrick to make sure he didn't have a ghostwriter.
Sheeran's retelling of the story goes, "I heard a very cool story about Eminem and Kendrick Lamar. Eminem...heard that Kendrick Lamar was the best rapper and he invited him to the studio to get him on a song. He arrived and Kendrick came with all his mates and Eminem said, 'I just want you in the studio, just you on your own. And then my engineer is gonna come in and then record you doing it. But your mates aren't allowed in...Everyone came in to listen to it and Eminem said that he did it to test Kendrick because he thought he had a ghostwriter."
Skip to the 7:50 mark to hear Sheeran tell the story.