What we know about Briony now

Briony clearly is most comfortable in a world that is "neat, limited and controlled" and totally understandable, which is why she is happiest with the written word that she can as if by "telepathy" transfer exactly what she means to her reader. It is the world of a child since it follows definite rules and for her there is a logical sequence, i.e. a drowning scene followed by a marriage proposal. When things do not follow this order, she does not understand and considers these events to be part of the adult world, which is clearly a different world than the world of a child. Briony longs to understand and be part of that adult world, but still a child and clearly not mature enough to be an adult, she just jumps to conclusions, without really questioning or taking the time to find the truth. The world is black and white for her and she assumes certain things right away (ie. that Robbie had somehow threatened Cecelia and had some power over her), which is why she does not have the ability to tell the truth later on. As she watches the scene between Robbie and Cecelia at the fountain, she believes that she is getting a glimpse of the adult world.
Briony watching the fountain scene is comparable to Briony's view of what happen when watching a play, as opposed to reading a novel, in which things can go wrong because of misunderstandings between what the playwright intended and what the audience observes. Briony totally misinterprets the scene she is watching, but does not realize it. As Briony watches Robbie and Cecelia, just as what can happen in a play, "the symbols were unravelled." As a result, she tumbles (as she states, "by chance") into the adult world and sets in motion what is no longer a life of "fairy tale castles and princesses," but one in which she "gets everything wrong" and as a result has total destructive power over Robbie's and Cecelia's lives.