David Harbour, star of Gran Turismo, Stranger Things, and of course the upcoming MCU project Thunderbolts, has said in a recent interview that the entirety of Black Widow's third act was reworked during shooting.
Speaking with Josh Horowitz of Happy Sad Confused, Harbour said: "What's so great about [Marvel] is they rework stuff even big productions like Black Widow... the entire third act was reworked as we were shooting, which is incredible. They just want that freedom."
Reworking a movie in the middle of shooting might sound like a disaster, but Marvel are known for switching things up. The MCU is always looking to avoid leaks, and so sometimes even scripts might not contain the whole truth of a movie. Black Widow's third act wasn't very well-received, though, which maybe points to sticking to the original plan being the best course of action.