Viola Davis channelled her eight-year-old self for her tough girl role in Suicide Squad - because that was the last time she was mean and nasty.

The actress plays sociopathic Squad leader Amanda Waller in the new comic book blockbuster and admits she struggled to find the character at first.

She had to go back over 40 years and reconnect with herself as a troubled kid - before her life got better.

She also remembered what she was like as a tough little girl, adding, "I tapped into Viola at eight, because I can't tap into Viola at 51.

"At eight, I could beat somebody! I was just always angry because people were always teasing me and I was bullied.

"That was the first story I told (director) David (Ayer) when I met him and he wanted to know about my childhood. I said, 'Well David, I remember when I was eight-years-old, I kicked a lot of ass!' Now, I'm always apologising. I'm shy and always retreating. I never tap into my power and Amanda Waller is not that. She is unapologetically brutal.

"I had to tap into that, otherwise I would've retreated and with this group I couldn't retreat."

Castmate Joel Kinnaman also helped out by giving her a book she found really useful for research: "Joel gave me a book called Confessions of A Sociopath and I read that book extensively," she tells WENN.

"One of the things in the book is a confession of a woman who is a sociopath, and one of the things I found out is women who are CEOs of companies are sociopaths. They have no guilt and if they cry they only cry because they feel like they're losing control."