What the title says - Peter Firth on the subject of Ruth/Harry, and Nicola, in this week's Radio Times. (Under a cut, although I don't think it's at all spoilery.)
"The romance was never scripted," admits Firth. "It was her and I flirting in takes, holding looks too long, lingering over lines. The writers picked up on that and it just grew from there. Nicola Walker, who plays Ruth, is a bit unpredictable - you never know what she's going to do - but she's an utter joy to play alongside."