Scarlett Johansson worked out hard for her role as the Black Widow in Iron Man 2 — but she found convincing herself she looked the part harder than training for it. The Hollywood beauty admits she was petrified about locking horns with costars twice her size. “I did a lot of stunt training, weight training and all that stuff for Iron Man,” she said. “It took strength training to try to fit that mold. I wanted to be able to look at something and go, ‘Okay, I’m going to knock the s**t out of that person.’ “To believe that is a challenge in itself, especially when you’re 5?3? and fighting someone who’s seemingly a giant. You have to be able to have conviction in that. That was just as much of a challenge as the actual physical part of it.”