Sarah Alexander plays Susan Walker

Sarah Alexander plays Steve's now pregnant girlfriend Susan.

Pretty and sexy, Susan's an unabashed go-getter. In fact her uninhibited attitude towards sex and relationships often catches the people around her unawares.

Writer Steven Moffat comments: "The others in the group perceive Susan as the super-confident one, although this isn't really the case.

"Similar to Patrick, she is fairly non-complex, and yet she is also strong, bright and clever. She may well have actually preferred to have found herself an Alpha man, got married and become a mother.

"Despite the fact that she's a successful career woman, she would never power-dress and really rather enjoys the fact that she's a fun-loving, perfectly happy girly-girl."

Sarah is very happy playing Susan: "It's great fun coming back because we really enjoy ourselves. My character is very grounded, sorted and confident in her sexuality.

"Her comedy comes from her directness, which means she sometimes puts her foot in it, but you always know where you are with her.

"She's a good-time girl who enjoys her life and her man. I've certainly learned a few lessons from her about self-belief, but we also catch glimpses of her being slightly insecure, out of her depth, which is very funny.

"Mind you," she adds, "Susan is very tolerant - I would have booted Steve out years ago!"

Sarah admits that she spends many hours scrutinising Sue Vertue, on whom her character is based, in rehearsals. But now the part is well and truly written for her.

Steven laughs: "I write with her in mind. It's her face, her voice. And I like taking the piss out of her scary eyes."

Sarah was recently seen in the hit BBC ONE comedy The Worst Week Of My Life and also stars in Green Wing for Channel 4.

She is also now penning her own comedy series, about journalists, with two writers from the Armstrong and Miller camp.