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What attracted you to the role of Viv?

When I first auditioned it was for Kay, but I knew I wasn't going to get the role because it just wasn't right for me. I was so surprised when I got a re-call and Richard Laxton – the director – said he wanted me to play Viv not Kay.

I was so excited by playing Viv, her vulnerability is hidden beneath this veneer of confidence in her sexuality, but also combined with a desire for male attention, making an incredibly complex character.

Viv finds it so hard to follow her heart, blocking herself so much; she clouds her choices because she doesn't give herself the time to breathe. For Viv there is the whole destructive relationship with Reggie and that seems to dictate her life and could dictate the rest of her life if she isn't strong enough to break away from it.

Why do you think she stays with Reggie?

Viv loves Reggie and when someone knows so much about you and has seen what he has seen her go through, it is terrifying to feel worthy enough as a person to start afresh with someone else. Viv believes she doesn't deserve anything better than what she's got with him – which keeps her with him.

I don't think it's a simple case of its just habit – it's more the fear of the unknown.

Do you think it is the war that keeps them together, or do you think they are in love?

I think the war brought them together in the sense that he is on leave, desperate to get home – and this frantic energy reflects their relationship. But I think war is the backdrop to a story about people and relationships. It's not a story about the war and I don't think that the war was why they were attracted to each other, but the fact that he was married, forbidden.

Unfortunately this was probably quite attractive at first and often people are hooked on something they can't have, with the promise of maybe having it. All their time together is exciting.

Then something happens – I won't reveal what as it will spoil the story – but it does cause their young love to be shattered.

How would you describe Viv's relationship with her brother Duncan?

Incredibly tested: there is unconditional love, but we disapprove of one another's life choices so much. It does weigh heavily on us, particularly in 1947 and the way he's choosing to live is not how Viv would want him to.

However, there is no way she can judge him because he knows about Viv's affair. Viv has seen Duncan at his lowest point in his life, and he knows that and it really is a classic case of siblings that feel they don't deserve anything more than what they have. It takes us both until the end of the piece to find the strength to move forward.

But I love the brother and sister relationship, they do have each other's back, regardless of the situation: the people you fight with most are the ones you love the most.

The drama – quite unusually – starts at the end and works backwards. What do you think this reversed narrative brings to the piece?

I think if this part of the book hadn't been emulated in the drama, then it would be a real loss. Sacrifices have to be made when transferring between a novel and a screenplay but I think to have not used this would really have been to miss a trick.

In the drama everything is revealed slowly and dramatically. It is hard to act though, one of my first scenes shooting was actually the last in the piece, so it was all quite confusing, I kept thinking: "Well where am I? What am I supposed to be feeling? What's happened in my life?"

It was interesting to know it and have gone through it but not give anything away. Viv holds everything together, and we see it through her eyes, but I think it's a great way to do a drama.