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.
Cast the Runner-Up: Joe Wright had wanted Keira Knightley to play the role of Briony in her late teens, but she immediately liked the character of Cecilia, and also wanted to get away from playing girls on the brink of womanhood and play a more mature character for once.
Dawson Casting: 25-year-old Romola Garai was cast as 18-year-old Briony.
Posthumous Release: John Normington, a veteran of stage and screen, has a minor role as a vicar, but sadly died before the U.K. release.
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.
Throw It In!: In the DVD Commentary, 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.