Olivia Wilde says her co-star had to be "buttered" and "squished" to fit into a race car for their new movie.

The actress stars in Rush, a biographical film which centres on racing in the 70s. It follows the life of champion drivers Niki Lauda, played by Daniel Bruhl, and James Hunt, played by Chris Hemsworth.

Both Olivia and Chris struggled when they tried squeezing into the speedy vehicles.

"I did get in the car. I could barely fit in the car. They're so small," she told MTV.

"We buttered [Chris] up and squished him in."

Olivia is full of praise for her co-star Chris and the rest of the cast and crew. She also feels fortunate to have worked with director Ron Howard.

"Ron Howard is obviously amazing, as is Peter Morgan who wrote everything," she said. "They were the Frost/Nixon team and they're amazing. I think, you know, it's an example of something I'm really lucky to be a part of, and watching Ron made me want to be a better director."

The 28-year-old also addressed the reason Russell Crowe didn't appear in the movie. The actor was in talks to play Richard Burton.

"The part ended up being so small that we felt guilty having Russell come play an extra in our film," she laughed.

"I think he would've done it. That's kind of a minor part, but it's an amazing part of this story... My character, who's a real woman named Suzy Miller, ended up leaving James Hunt for Richard Burton who left Elizabeth Taylor for her, and it was a huge scandal. So, yeah, we didn't end up making Russell Crowe come and stand in the shadows for four minutes [laughs]."