But the funniest part of the segment came when Corden brought up a subject near and dear to Mendes' heart -- Harry Potter.
"On a scale of ten being crazed superfan and zero is, hate it, never seen it, where are you?" Corden asked Mendes as they drove.
"9.5," Mendes responded without having to think about it much. "I have a whole book at home that I wrote all the spells in and stuff."
As they continued discussing his obsession, Mendes looks more and more different as the camera cuts back and forth between the two. First, the telltale lightning bolt scar appears on Mendes' head. Then, he's suddenly wearing Harry Potter's trademark wire-rimmed glasses.
"Have you ever met any of the cast of the movies?" Corden askes.
As the camera cuts back to Mendes, he's in full-on Hogwarts robes (Gryffindor colors, naturally), holding a wand, talking about being
starstruck (and sweating) when he met Emma Watson, who played Hermione Granger.
"So you're like a proper superfan?" Corden asks.
"No, I mean, I'm like a fan, I like it, but I'm not like crazy obsessed with it," Mendes responds, as it's revealed he's gone full Dumbledore with purple robes and silver flowing hair and beard.
See the whole hilarious segment below.
For a look at Mendes' recent collaboration with one of his musical idols, watch the video below.