These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Alternate Character Interpretation: Briony's actions regarding the happy ending she gives to Robbie and Cecilia in her novel: is she genuinely making amends to people she wronged the only way she can and knows how, or is she selfishly, cravenly and dishonestly attempting to rewrite history and cover up her own actions to make herself feel better, and hiding behind atonement as a justification?
Genius Bonus: After Briony has accused Robbie of raping Lola, she is shown standing in front of a stained-glass window of St. Matilda, the patroness of the falsely accused, and also the subject of a nursery rhyme that begins "Matilda told such awful lies/ She made one gasp and stretch their eyes..."
Tear Jerker: When Briony reveals that what we have been watching was her book with a falsified ending and that Robbie and Cecilia died and never had their chance of happiness entirely because of her actions. She grants them in fiction what she denied them in life. It's the only way she could deal with the guilt of what she had done.
What an Idiot: Cecilia knows Briony has an overactive imagination. She's clearly afraid of Briony showing the letter to her mother. You'd think as soon as everyone was looking for the twins, she'd at least take Briony aside and explain the situation.
Cecilia and Robbie never consider Marshall as a possibility for the real rapist until Briony tells them, even though he was the stranger in the house that night and had showed up at dinner with a scratch to match Lola's bruises. (In an interesting example of Not So Different, they seem to have settled on Danny Hardman as certainly as Briony settled on Robbie, and based on as little evidence, prompting Robbie to remark that they "owe Able Seaman Hardman an apology.")