Bill Murray has become an official part of the MCU. He is an unrevealed character in Ant-Man and the Wasp: Quantumania. It is assumed he will be a villain of some sort for the movie.
In the past, he hasn’t been interested in being a part of a comic book movie. Murray joined the cast because of director Peyton Reed. Murray is a fan of Reed so Reed was able to convince him to join the cast.
(Via Comicbook.com)
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