Rather like Trillian in The Hitchhiker's Guide to the Galaxy, on paper Clare Keightley isn't perhaps the most vividly written of characters, and I knew I needed an actor who would bring a real inner strength to the role.

I've admired Susannah Harker's work ever since that wonderful serial House of Cards, and her subsequent appearances in dramas like Pride and Prejudice and Ultraviolet have confirmed that she's a performer of genuine power and substance. I approached Susannah's agent hoping against hope that she'd be interested in the role, and it turned out that she was something of a childhood fan of Doctor Who!

If I recall correctly Susannah was the first of the guest cast to confirm, and she gives a terrific, spirited performance. She had particular fun with Clare's ongoing quest for her missing bicycle!