At one moment, I found myself doubting the reality of my own adventure; at another, I was perplexed and distressed by an uneasy sense of having done wrong, which yet left me confusedly ignorant of how I could have done right. (22.214.171.124)
The style here, with the use of clauses and repetition, helps emphasize Walter's distressed mental state after his encounter with Anne Catherick. She does seem to have that effect on people.
The matter begins and ends with the boy's own perversity and folly. He saw, or thought he saw, a woman in white, yesterday evening. (126.96.36.199)
Little Jacob's "lie" (which isn't really a lie) introduces some thematic ideas of seeing and perception… and how shadowy the truth often is.
I saw it in her white face; I saw it in her trembling hands, I saw it in her look at Laura.
After waiting an instant, she turned from me in silence, and slowly walked away.
I closed the door again. "O Laura! Laura! We shall both rue the day when you called the count a Spy!" (188.8.131.52-4)
Turns out the master of deceit is a bit sensitive about being called out on it, which gives us a lot of insight into his and his wife's characters.
"I do understand it, Laura. He is mad—mad with the terrors of a guilty conscience." (184.108.40.206)
Sir Percival is basically a portrait of the psychology of guilt and the effect it can have on someone's behavior. Don't tell the truth because it's morally correct—tell the truth so you don't have to suffer guilty-conscience-based madness!
My distrust of his unfathomable falseness, my sense of my own degradation in stooping to conciliate his wife, and himself, so disturbed and confused me, that the next words failed on my lips and I stood there in silence. (220.127.116.11)
This powerful passage gives us great insight into Marian's turbulent emotions. The diction here really makes Marian's disgust and hatred of Fosco palpable. Another reason to not lie: Marian (the world's coolest woman) hates liars.
You have taken your own mean, underhand view of the innocent deception practiced on Lady Glyde, for her own good. (18.104.22.168)
Sir Percival's lame defense of the "deception" he uses to get Laura out of the house emphasizes what a creep he is. But what's interesting is that Walter and Marian later practice an "innocent deception" of their own against the ever-helpless Laura. We could kick-start a really interesting debate about intentions and the morality of lying here.
Nothing that he had said or did shook my opinion of the disgraceful series of falsehoods that he had told, in my presence, the day before, or of the cruel deception by which he had separated Lady Glyde from her sister. (22.214.171.124)
Mrs. Michelson comes down pretty harshly on the side of truth here, as her diction reveals. She never forgives Sir Percival for the lies he told and the way he treated Marian and Laura.
Her mind in this instance […] confusedly presented to her something which she had only intended to do in the false light of something which she had really done. This unconscious contradiction of herself was easy to account for in this way—but it was likely to lead to serious results. (126.96.36.199)
The unreliability of memory is a running theme throughout the book. Here Laura becomes a sort of unintentional liar, thanks to her Swiss cheese memory, which confuses fact and fiction.
It was hard sometimes to maintain our innocent deception, when she proudly brought out her purse to contribute her share towards the expenses, and wondered, with serious interest, whether I or she had earned the most that week. (188.8.131.52)
Contrast Walter's description of his "innocent deception" of Laura, with Sir Percival's (featured above), which is terse, abrupt, and defensive in tone. In case we weren't all on the same page, Walter is cool, Sir Percival is a punk.
The conception involved nothing less than the complete transformation of two separate identities. Lady Glyde and Anne Catherick were to change names, places, and destinies, the one with the other. (184.108.40.206)
Even Fosco seems overwhelmed by the prospect of switching two people's lives and identities. Swapping Laura and Anne is the book's greatest lie and its most significant commentary on identity issues.