"That one is directly from Chris. You know, it's his mate," director Taika Waititi told ET of how they convinced [SPOILER!] to join the Marvel Cinematic Universe.
Spoilers below for Thor: Ragnarok.
If Matt Damon's involvement in the Thor: Ragnarok press cycle (his "surprise" appearance on the red carpet at the premiere, that time he crashed Chris Hemsworth's Jimmy Kimmel interview) has seemed stranger than, say, a doctor-turned spell-casting sorcerer supreme, there's a reason. The reason is that Damon is in the movie.
If you recall, at the end of Thor: The Dark World, Loki (Tom Hiddleston) does away with his father (played by Sir Anthony Hopkins), then assumes Odin's identity to take over the throne. If you don't remember the plot of The Dark World, there's an early sequence in Ragnarok in which Thor returns to Asgard to find Loki-as-Odin enjoying a play that recaps the climax of the last movie, where Loki seemingly dies a heroic death. (Essentially, this scene.)
In that Asgardian theatre troupe, the role of Thor is played by Hemsworth's older brother, Luke; Odin is played by Sam Neill, who starred in director Taika Waititi's last film, Hunt for the Wilderpeople (and who happens to be a fellow New Zealand native); and Loki is played by Damon in a jet-black wig.
"That one is directly from Chris. You know, it's his mate," Waititi told ET of how the Damon cameo came to be. "I know of a couple other movie stars, and I'd asked one of them. He was keen, but then he was doing some other thing, so he couldn't come. Boring story now, 'cause we can't say the name."
MORE: 'Thor: Ragnarok' Director Taika Waititi Talks His 'Surreal' Scene-Stealing Role as Korg (Exclusive)
As for how Damon and Hemsworth know each other in the first place? It's not from having worked on any movie together, as this is the first time they've actually shared screen time. In a 2014 GQ interview, Hemsworth said, "We became friends around the time I started to work and I’ve really benefited from watching how he handles himself. Matt's just a normal guy who has the movie-star thing figured out."
They really are, it seems, just mates. Damon said during a 2016 radio interview, "I'm really tight with him and his wife and the kids." The two families have even vacationed together in the French Riviera and Australia, and the Damon brood makes regular appearances on Hemsworth and wife Elsa Pataky's Instagram accounts:
Marvel has done its best to keep Damon's cameo in Thor: Ragnarok a secret, even excising his name -- as well as Hemsworth and Neill's -- from the movie's press kit. (Only Charlotte Nicdao is credited as "Actor Sif.") Now that the movie is in theaters, Waititi can finally praise Damon for "coming in and crushing it" and says that locking down the Oscar winner and Luke Hemsworth, especially, "was a win, that day when we got those two."