The rule doesn’t stem from the fact that Reynolds, not Deadpool & Wolverine director Shawn Levy, is the movie’s true auteur. Rather, it comes from an understanding of the stakes of the movie.
“If everyone [broke the fourth-wall], then you would no longer invest in that character as much. You really want to believe that the villain is a villain. You really want to believe that your costar’s character is true as well. Deadpool can undermine that—and does undermine that—because you don’t want the audience to take him as seriously.”
The restriction keeps the characters true to the comics, as most people in the Marvel Universe treat Deadpool’s meta textual commentary as just one more irritating thing that he does. That response sets Deadpool aside from other fourth-wall breaking comic book characters. When, say, Ambush Bug in the DC Universe or even his fellow Marvel Universe denizen Howard the Duck, know about the real-world, they do so almost without any acknowledgement from anyone else in the scene. But Deadpool uses it as a strategy, a way of coping with his madness and being very good at killing people.
If everything within the world were to feel fake, it would cheapen everything, which Reynolds understands… and Jackman did not. At least at first.
“I made the same mistake—day one. I made the suggestion and it was like, ‘Yeah…’,” admitted Jackman. But after Reynolds explained the importance of the restriction (“I always feel like an absolute dick when I’m trying to explain: I need people as invested as humanly possible in every other character while knowing that only Deadpool is the unreliable narrator,” he said), then Jackman got on board.
“Ryan beautifully describes Deadpool’s brain as a half-baked omelet,” continued Jackman. “And so, whatever he’s doing—talking to a camera, to Wolverine—it’s just another layer of annoying crap that I’ve got to put up with. Who knows what the hell he is doing? But it’s just another excuse to punch him in the face.”