Mission: Impossible – Fallout director Christopher McQuarrie was a guest on Empire’s podcast to talk about the movie, and he went into great detail about the whole Justice League situation.
McQuarrie said that he got a call from Justice League producer Charles Roven, asking if they could shave Henry Cavill’s mustache for their reshoots. McQuarrie was eager to help, but had to talk to his team to make sure the move made sense for everyone.
Someone from the Warner Bros. side had suggested that they would pay to digitally add Cavill’s mustache back into Mission: Impossible. McQuarrie was against the idea because a fake mustache would never look as good as the real thing, and his producer Jake Myers calculated that they would need $3 million from Warner Bros. to add a digital mustache to Cavill.
So the compromise was this — McQuarrie was willing to take the $3 million from Warner Bros. to shut down production on Mission: Impossible until Cavill was able to grow his mustache back after doing the Justice League reshoots. But when that compromise was presented to Paramount Pictures, the studio that was making Mission: Impossible, the idea was thrown out.