Scarlett Johansson has revealed that she had to "grin and bear" the intense training she had to undergo to play Black Widow in Avengers Assemble.

The 27-year-old actress had to learn Wushu and handle some new weapons as she reprised her role as Russian spy Natasha Romanoff and her superhero alter-ego from Iron Man 2 in Joss Whedon's blockbuster.

"You just grin and bear it. You do a lot of strength training and a lot of weight resistance training and you do it at 4am, you go to work and you treat yourself to a big, tall, delicious glass of water," she said.

"I have to be honest, the first time I saw what they had in mind I was like, 'I'm never going to be able to learn this.' So it's just a lot of failures until you get it right, but boy do those failures hurt sometimes!"

Scarlett is thrilled that a peek into the Black Widow's background is shown in Avengers Assemble, which is in cinemas now.

"I think that darker side was always kind of looming there, but it's nice to be able to have the opportunity to reveal a bit about that past," she said.

"She's a mercenary, she's a killer for hire - or she was - and it's always interesting to know how she got to that place. And of course, she's salvaged by Shield in a way. There's a lot of back story there, but I do like the fact she's not righteous."