Young Briony is played by Irish actress Saoirse Ronan. Romola Garai who plays the adult Briony played a Fake Irish in Rory O'Shea Was Here. And she was also involved in a spot of unrequited love involving... drumroll please... James McAvoy. In both films she also plays a girl who spontaneously enters a caregiving job - she also dresses up as a nurse for Halloween in Rory O'Shea Was Here.
An odd one. Keira Knightley previously died in reality but the witness responsible for her death created a happy ending for her in The Hole.
Award Category Fraud: An uncommon variant. Keira Knightley campaigned as lead but ultimately failed to place at the Oscars. While it can be argued that she was a co-lead (and marketing materials made sure to sell her as one-half the face of the movie), Saoirse Ronan was submitted (and snagged a nomination) as Best Supporting Actress. Briony is in no way a supporting character, especially to Cecilia, but Ronan was relegated to the supporting categories largely because she was usurped by more famous actors, her young age, and the fact that, much like Lion, she plays only one part of Briony's life, while Keira Knightley is always Cecilia.
Billing Displacement: Saoirse Ronan (and all other iterations of Briony) is usurped by Keira Knightley and James McAvoy so often that she tends to warrant only a tiny fraction of film posters and promotional material, which milk the more famous stars' faces for all their worth (as Ronan wasn't famous yet). Though the plot centers on Cecilia and Robbie and their doomed romance, Briony is still unquestionably the main character who the narrative follows the closest. See Award Category Fraud above.
Dawson Casting: 25-year-old Romola Garai was cast as 18-year-old Briony.
Fake Brit: Irish Saoirse Ronan as English Briony Tallis. Averted with Romola Garai and Vanessa Redgrave, who play Briony's older selves - as they are both English. James McAvoy is a borderline case. He's Scottish, doing an English accent.
Playing Gertrude: Romola Garai is three years older than Keira Knightley, who plays her older sister. It's especially notable in the one scene they share together, because Cecilia doesn't seem to have aged at all. Although this actually makes a weird kind of sense: since Briony imagined the entire scene for her book. She thought of herself as much older and more mature after her time as a nurse, but she was still thinking of the Cecelia from her childhood, since in real life they didn't reconcile. But this was mostly just a lucky fluke of having one actress play Cecelia while three played Briony.
Playing with Character Type: Invoked by Keira Knightley. She was approached to play the adult Briony but asked to play Cecilia instead - saying she'd had enough of "coming of age ladies".
The Red Stapler: The gorgeous green dress worn by Keira Knightley is practically iconic. Replicas of varying quality can still be bought in several online stores.
Serendipity Writes the Plot: The Dunkirk scene wasn't supposed to be a Oner. It had to be one because all the extras can't stay for very long and the environment conditions gave a very short window for filming, so Director Joe Wright came up with the oner idea so everything can be done in one go.
Star-Making Role: For Saoirse Ronan. She was cast in The Lovely Bones specifically because Peter Jackson wanted an unknown as Susie. Then this film came out and she got an Oscar nomination.
Throw It In!: In the DVD commentary, Director Joe Wright reveals a lucky fluke that got caught on camera during the scene just before Robbie discovers the school girls massacre. At the point where he removes his helmet, the weather was cloudy. As he looks up the sky, the sun started shining and then got cloudy again the moment he put his head down.
What Could Have Been: Abbie Cornish was strongly considered to play the adult Briony. But she was busy with Elizabeth: The Golden Age.