Thompson is equally vociferous on matters of faith. For Waugh, a Catholic convert, the central theme of Brideshead Revisited is religion (or "the operation of divine grace on a group of diverse but closely connected characters"). Thompson is with Charles Ryder on this one.

"I'm an atheist; I suppose you can call me a sort of libertarian anarchist. I regard religion with fear and suspicion. It's not enough to say that I don't believe in God. I actually regard the system as distressing: I am offended by some of the things said in the Bible and the Koran, and I refute them."

She knows she's being controversial, but she believes passionately in what she says, and passionately believes it needs saying.

"I think that the Bible as a system of moral guidance in the 21st century is insufficient, to put it mildly," she continues, frowning a little.

"I feel quite strongly that we need a new moral lodestone if we can't rely on what is inside our own selves. Which I think, actually, is pretty reliable."

Taking the role of Lady Marchmain has probably strengthened her convictions.

"I've never played anyone as extreme," Thompson agrees. "I think she was brought up by people who withheld all affection in lieu of the love of God, damaging her emotionally, which she then carried into her own parenting, and her children are damaged, too.

"She means well, though," Thompson says, leaping instinctively to her character's defence. "She really does.

"And I'm sure everyone's souls were fine, with or without her."