Sometimes these things come from the most unexpected places and from industry veterans who really do not care about Marvel Snipers. The last time we had a leak like this it was from Alfred Molina just telling the world he was returning as Doc Ock to a reporter who was not expecting it all. We can only imagine that this reporter, Patrick Heidmann, was not expecting to find out that Bill Murray is going to be in a Marvel movie and get the specific film, Ant-Man, and the Wasp: Quantumania, while interviewing him about The French Dispatch. That is exactly what happened, or at least that appears to be the case according to this translation and Heidmann himself on Twitter.
Interviewer: "Is it especially important for you to work with people you know well and like?"
BM: "You know, recently I made a Marvel movie. I probably won't tell you, but never mind. In any case, some people were quite surprised why I decided on such a project. But for me the thing was quite clear: I got to know the director – and really liked him very much. He was funny, humble, everything you want from a director. And with the cheerleader story "Bring It On – Girls United" he made a movie years ago, which I think is damn good. So I agreed, although I'm not interested in these huge comic book adaptations as an actor otherwise."
Interviewer: "And did you regret the decision?"
BM: "Let's put it this way: the director is a good guy, and now I've at least tried out what it's like to shoot a Marvel movie. But I don't think I need that experience a second time. And to come back to your previous question: I have mostly had a pretty good knack for avoiding the nasties of this industry. Fortunately, most good artists are also good people. At least that's how I experience it with my friends."
We know it's Antman and the Wasp: Quantumania not just because the movie is in production, but Murray cited Bring It On as another movie that this director worked on. Peyton Reed was the director of Bring It On. So if the math is correct, Bill Murray is in the Marvel Universe now because he really liked a cheerleading teen comedy from the year 2000. The fact that he says he doesn't want to do it again means this is probably a one-off role or cameo. Marvel is one of the biggest games in town, and even longtime veterans of the industry are wondering what it's like to work with them.
Ant-Man and the Wasp: Quantumania is directed by Peyton Reed from a screenplay by Jeff Loveness and stars Paul Rudd as Scott Lang and Evangeline Lilly as Hope van Dyne, alongside Michael Douglas, Michelle Pfeiffer, Kathryn Newton, and Jonathan Majors.