How Saoirse Ronan Became The Go-To Heroine For Ian McEwan Adaptations—from 'Atonement' to 'On Chesil Beach'

Saoirse Ronan stars as Florence Ponting in ON CHESIL BEACH, a Bleecker Street release. Robert Viglasky/Bleecker Street

On Chesil Beach is less than 40,000 words, but each is as polished and integral to Ian McEwan's novella as the famous pebbles that make up the 18 miles of protected beach along England's Dorset coast. Its moody beauty is the honeymoon backdrop for McEwan's 2007 book, in which a brief marriage will end after a night of disastrous sex. McEwan's pace is brisk and his tone often comic, at least for the reader; for the lovers, Florence Ponting and Edward Mayhew, the story will end in crippling shame and regret.

McEwan has written several original screenplays but finds the process exasperating and protracted, so he's let others adapt his best-selling novels into films: The Comfort of Strangers (1990), Enduring Love (2004) and Atonement (2007). He felt differently about On Chesil Beach, mostly because it's so short. "You can more or less stay with what you've got," the author says, "rather than cutting two-thirds of the book."

When theater director Dominic Cooke, who is making his feature film debut, suggested Saoirse Ronan for Florence, McEwan was delighted. The Irish actress played the disruptive younger sister in Atonement, for which she received her first Academy Award nomination. "Even then, she showed an extraordinary ability to take control of the material, to morph from a sparky Irish kid into an upper-class English girl, with cut-glass accent to match," ­McEwan says. "She knew how to convey a particular form of inwardness, of an imagination running riot. She also completely charmed and hypnotized the set."

For her part, Ronan, 24, was eager to jump into another McEwan creation. "Ian is someone who writes women incredibly skillfully, with a well-rounded perspective," she says. "I'd like to play another one written by him every 10 years."

Though Florence and Briony have little in common, they both come from repressed worlds where much is left unspoken. And Ronan, McEwan says, "is one of those actors who, just with a silence, can show you a thought process in her face." Her mismatched husband, Edward, is played by Billy Howle, who had just worked with Ronan in Michael Mayer's adaptation of Chekhov's The Seagull, due in theaters May 11.

McEwan's story is set in 1962, in pre-sexual revolution England. Florence, 22, is a gifted violinist, the daughter of an unloving mother (an Oxford don) and a successful businessman. "It's an icy household," says the author. While Florence is commanding in her work as the leader of a classical string quartet, on her wedding night she is still sexually innocent and tormented by fear. "Saoirse has gifts of immediate emotional insight that any novelist must envy," says McEwan. "She can make herself the ordinary girl next door and then, by a shift of expression, a turn of the head, assume great and unusual beauty. But above all, it's her intelligence that I cherish."

More than 10 years have passed between Atonement and On Chesil Beach, but McEwan was astonished by how little the years—and two more Oscar nominations, for 2015's Brooklyn and last year's Lady Bird—had changed Ronan. "Burgeoning fame hasn't swamped her," he says. "She remains firmly herself, and it's a lovely self, in which the natural 12-year-old child still retains a fun-loving role."

On Chesil Beachopens in theaters May 18.