Even the usually excelent actress Scarlett Johansson.
Sorry. Can't take you seriously anymore.
She has, like, one facial expression. Always! How can she be excellent?
I mean, if you take away her beauty, what's left?
Why do you think they didn't show her "cry" in her scene with Loki? Because, most likely, she can't do it convincingly.
Have you seen Lost In Translation by any change? You say that she most likely can't do it convincingly. I'd strongly suggest you watch Lost In Translation.
And beyond that, she has done other films in which she has proven herself a good actress.
Take for instance Natalie Portman, who was great in Black Swan and has done numerous films that establish her as a character actress. However, she was horrible in the Star Wars prequel. I don't think it's fair to judge an actor or actress bad because of one film. Most of the actors/actresses we idolize will have atleast one of two films in there repetoire in which they kinda stunk. Why? Who knows. Bad directing, they couldn't identify with the character... so many reasons.
And I know a lot of people would say that said actor/actresse should just leave the film then. Not so easy I'm afraid. They signed contracts, and it would cost them dearly to just up and leave. And in the end of the day, acting is just a job and you need to get paid. So you don't just leave a paying job.