I thought, No one is going to believe I’m a police officer. This is ridiculous

When the writer David Hare asked Carey Mulligan to play the part of Detective Inspector Kip Glaspie, a highly motivated and enormously capable police officer in Collateral, his absorbing new four-part drama for BBC2, the modest actress confesses that initially she thought she might not be up to the job.

Mulligan, who has in the past tended to concentrate on period dramas such as Pride & Prejudice, The Great Gatsby, Far from the Madding Crowd and Suffragette, remembers that, “Right at the beginning, I thought, ‘No one is going to believe I’m a police officer. This is ridiculous. They have such authority, and I’m scared of them!’

“I was also pregnant during filming, so the first time I put on a SOCO [Scene of Crime Officer] suit, I looked like a Teletubby! I was also very self-conscious about pulling out the police badge. I said, ‘This is silly. No one’s going to buy this!’”

So the 32-year-old actress, who is married to Marcus Mumford from the band Mumford and Sons and has two children with him, took the unusual course of trying to persuade herself that Hare was right to offer her the role. “It was about convincing myself that I was grown-up enough to portray a police officer.

“I still think I’m 21 and an ingenue, but actually I’m a grown-up now. So maybe I could have a proper job!”

We should all be grateful that Mulligan persisted for she gives a tremendous performance as Kip, displaying real magnetism as the tenacious police officer.

Collateral is set over four days in contemporary London. After the fatal shooting of a pizza delivery man, the unexpected ramifications of the crime soon start to spiral out of control.

At first, it looks as if the shooting is a random act of violence, but Kip refuses to accept that interpretation. She is dogged in her attempt to find out if a darker conspiracy lies beneath the murder.

The story also drags in other characters who are orbiting round the homicide. These include Romola Garai and David Mars (John Simm, Life on Mars). He is a politician embroiled in the aftermath of the crime due to his tumultuous relationship with his troubled ex, Karen (Billy Piper, Doctor Who), who was waiting for the fateful pizza.