So we're back to Briony. Her frustration mounts as the hours stream by without constructive rehearsal of what we can only assume is a masterpiece. She sees the whole Cecilia in the fountain scene from the window but can't hear it. Her imagination runs wild. And, with no knowledge of the world of adults, she assumes this is some sort of custom, a ritual, she's yet to learn about. Of course, she wants to write about it. That's almost definitely going to lead to trouble.
Interestingly, we get the "six decades later" in this chapter so we know absolutely that Briony lives to be an old lady, she's a successful writer, and her stories are amoral. I get the sense that the story (at least the Briony chapters) are written from the perspective of adult Briony, recalling the events. She's very self-analyzing and not a lot of credence is given to the factuality of the events or timeline.
On the other hand, Cecilia's chapter had an of-the-moment feel. She's not as reflective and more turmoil is lent to her thoughts and actions. There is no distancing or objectivity about the life of Cecilia. We are given no clues about her future or Robbie's. We don't even know if she later regrets her actions at the fountain.