That hair – you could pick Alex Kingston out of a crowd of hundreds thanks to her distinctive crescent of curls. It makes her simple to spot in a theatre cafe in London, slugging bottled water during a break in rehearsals of her new play.

She's performing at the Donmar in Luise Miller, an 18th-century work by Schiller that's not often staged in the UK. The six-hour running time sees to that, but Kingston will perform in a streamlined translation: two hours. The story, she explains, "is like Romeo and Juliet without the early frisson. We meet our lovers already crashing and burning. That's Schiller for you!"

The 48-year-old is a declaimer, her voice rich and throaty, the result, surely, of her time at the RSC. She shifted focus to TV in the 1990s, the title role in ITV's Moll Flanders leading to the offer of a part in US drama ER. Newly separated from first husband, Ralph Fiennes, she moved to LA and gave up the stage. "My CV is very short. There wasn't time for anything."

The character she played on ER wasn't dull, exactly, but wouldn't top many fan-favourite lists either. Dr Elizabeth Corday was a stiff British surgeon: wild corkscrew hair belying repressed emotions, plus other dreary anglo-cliches. Not a lot of scene-stealing opportunities over 10 series.

All change in her latest TV job. As a recurring character in Doctor Who, Kingston steals scenes; she makes off with entire episodes. Matt Smith "gets very jealous," she says, "as I get all the best entry and exit lines." And she does, playing a time-travelling adventurer with a tantalisingly vague backstory. Kingston actually died on screen in her first appearance, in 2008. "After that, I thought, job over." You forgot about time travel, I say. "I forgot about time travel!" So she's been returning for years, grateful for the liberties of science fiction.

She howls when I tell her the chemistry between her and Smith has become almost indecently sexy for a kid's TV show. "Matt's a great flirt. But I feel… I'm going to make myself sound old, but I feel quite motherly towards him." Smith is 28. "Maybe not motherly. Actually, I hope he doesn't regard me as a mother." She's still chortling and shaking her water bottle. "Older sister, perhaps."

She has a 10-year-old-daughter, currently being looked after by her dad (Kingston's second husband) back in LA. "We spoke on the phone last night and she told me she'd got six school friends hooked on Doctor Who." Get her on the BBC payroll, I suggest – that's six lunch boxes sold.

Kingston clenches her fist; the curls bounce. "Kerching!" she shouts.