There is no reason to believe she has been cast in the film. Stranger Things Season 2 is filming very close to where Avengers: Infinity War and Black Panther are currently in production. Brown likely hopped over from the studio Stranger Things is shooting at to Pinewood Studios to check out what the Russo Brothers are putting together in the sure-to-be epic ensemble film releasing next year.
Safe to say, Brown is living the dream right now. Sitting side by side with the directors of one of the largest and most anticipated movies in history is something Tom Holland couldn't even fathom when he watched The Avengers in theaters back in 2012.
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Avengers: Infinity War is described by Marvel Studios as the culmination of everything in the Marvel Cinematic Universe since Iron Man in 2008. The movie is filming now under directors Anthony and Joe Russo, from a screenplay by Christopher Markus and Stephen McFeely. The expansive confirmed cast includes Robert Downey, Jr., Chris Evans, Chris Hemsworth, Mark Ruffalo, Jeremy Renner, Scarlett Johansson, Tom Holland, Elizabeth Olsen, Paul Bethany, Sebastian Stan, Benedict Cumberbatch, Benedict Wong, Chris Pratt, Zoe Saldana, Karen Gillan, Vin Diesel, Dave Bautista, Bradley Cooper, Pom Klementieff, and Josh Brolin as the villain Thanos. Avengers: Infinity War and its as-yet-untitled sequel are filming back-to-back for a May 4, 2018 and May 3, 2019 release, respectively.