But if Tom Cruise had gotten his way, the Fallout trailers would have established that Walker was John Lark from the get-go.
August Walker was brought into Mission: Impossible – Fallout as the Special Activities Division agent assigned by CIA director Erica Sloane to keep an eye on Ethan Hunt and his IMF team while they tried to track down the plutonium cores stolen by the Apostles, who intended to sell them to fundamentalist John Lark.
So if you paid attention to Fallout’s marketing, you might have gotten a sense that although they were working together, Walker and Ethan would not get along, as least for the first chunk of the movie.
Christopher McQuarrie added during his interview on The Empire Film Podcast that Paramount decided instead to try to hide the fact that August Walker was one of the main villain, at least for the first trailer or two, due to spoiler concerns.