With all the hard work that goes into Tom Cruise's amazing stunts, the Mission: Impossible franchise looks like some pretty serious business. However, when Simon Pegg is around, you can be sure that a bit of fun is being had. In at least one case the target of his fun was the new guy to the franchise, Henry Cavill. In an interview that's part of the bonus features of the new Blu-ray release of Mission: Impossible - Fallout, Pegg reveals that he had a laugh at the expense of the Superman actor while the two were involved in filming the Paris car chase sequence in the movie. According to Pegg...
I'm the back with Ving and Henry, both of whom are large men. At one point Henry decided he didn't want to ride back in the car because it was quite cramped so he said 'I'll run up there.' And we just drove the car next to him running, and I sang the Supreman tune out the window at him.
While Simon Pegg doesn't specify, I'm assuming he was singing the classic John Williams Superman theme rather than anything from the current crop of DC movies. That's the one most of us would be thinking of when we think of Superman music, and the theme was used a little bit in Justice League. It probably sounded great with Henry Cavill running next to the car. He basically looks like Superman even when he's not wearing the suit, so one of the best themes in the history of film probably should just play in the background of whatever he's doing at all times.
It's not mentioned what Cavill thought of the joke. He seems to be a guy with a sense of humor, so I would guess he thought it was funny.
The scene in question sees the IMF team trying to escape the streets of Paris in a small BMW. Simon Pegg, Henry Cavill, and Ving Rhames all end up in the back seat together and while they do fit, it's a bit cramped. Simon Pegg is probably what you would call a "normal sized" human, but Cavill and Rhames are just big dudes, making the back seat a little snug. It seems that, rather than ride around in the car when they didn't have to, Cavill decided to stretch his legs.
Of course, the role of Superman and Mission: Impossible actually have a lot more in common than simply Simon Pegg humming the theme song on the set. While it was potentially unknown at the time Simon Pegg was poking some fun at Henry Cavill, his job as Superman would be substantially impacted by his Mission: Impossible job, specifically the facial hair he grew for the latter movie.
You can see Superman and Ethan Hunt together in Mission: Impossible - Fallout which is now available on Blu-ray and Digital.