Cate Blanchett, who played the villain Hela in Thor: Ragnarök, claimed recently in an interview with GQ that Marvel tends to work with unfinished scripts and that film production usually starts after they’ve sketched out the action sequences. From there, she said, they work backwards to construct the story. Blanchett, however, enjoyed working on the film and doesn’t necessarily see it as a negative, although there are certainly actors who have felt that way.
“Marvel aren’t necessarily used to having a whole script. Often, they will board the essential battles and then they’ll start working backwards and construct their stories.”
We have previously heard that Taika Waititi improvised a lot when he made the sequel, Thor: Love and Thunder, and in that case most people agree that it was not very successful. Even Chris Hemsworth has said in retrospect that they messed up and took it all a step too far.