The cast of a new BBC Two drama has been filming on location in Cambridge.
Page Eight tells the story of an MI5 operative's attempts to investigate a threat to the stability of the country's security services.
Director and screenwriter Sir David Hare spoke to the BBC from the set at Jesus College, where he studied in the late 1960s.
"It's hard to know what goes on in MI5, but this film takes an educated guess at what they must be up to," he said.
Speaking to BBC Radio Cambridgeshire presenter, Mandy Morton, Sir David said the premise of Page Eight was based on "informed guesswork".
"I've talked to people in MI5. People will always talk to playwrights much more freely than they talk to journalists because not only is it off the record, but it's also non-attributable.
"They know that whatever they tell me is going to be buried deep within the drama."
The film's lead character, Johnny Worricker, is played by Bill Nighy who, together with Rachel Weisz, filmed scenes at the college and in the city's Market Square.
The cast also includes Ralph Fiennes, Michael Gambon, Ewen Bremner and Judy Davis.
"This film is almost like a repertory company of actors I've loved and admired for years," said Sir David.
He described his screenplay as having the discipline of a musical score.
"That implies that it's terribly limiting for the actor, but it isn't. Any classical musician will tell you that you learn the score, and then you're free.
"So, beyond discipline lies freedom," he explained.
"The joy of this film is having people like Ralph Fiennes, Bill Nighy and Michael Gambon, with whom I've worked a lot, come to my work knowing exactly how to do it."
'Dowdy and routine'
Page Eight is being filmed for the BBC, an organisation Sir David has been critical of in the past, saying it was unimaginative and that it had lost its nerve.
He is hopeful that Page Eight will help to redress the balance between the BBC's fiction and non-fiction programming.
"I feel historically they made a huge mistake by turning themselves into a news organisation," he said.
"They staked the house on the idea that the government would not be able to destroy the world's largest newsgathering organisation.
"I think it was a political ploy in order to guarantee their own survival, but it's come at immense cost to their other strands, and their role as patron of the arts."
He continued: "To be fair, I think they have begun to realise that the balance of BBC output has become badly skewed, and in the last 10 years their fictional work has become dowdy and routine.
"However, having said that, I have to admit that they're making my film, so it can't be all bad."
Page Eight will be broadcast on BBC Two later this year.