"I shall never forget the weekend Laura died."
In an irony of ironies, the film opens with the narration of a man who by the end of the film is dead talking about the weekend that a woman who died who turns out to be very much alive at the end of the film. Odd, perhaps, unless reads the last half of the film merely the wish-fulfilling dream of a sleeping detective.
"I can afford a blemish on my character, but not on my clothes."
Another bit of irony, actually. Shelby cannot literally afford any blemish on his clothes because he is a social climber with no money to call his own. For this exactly reason, of course, Shelby cannot afford a blemish on his character, either.
“I don't know a lot about anything, but I know a little about practically everything.”
What seems a bit of self-deprecating honesty is, ironically again, actually a bit of self-aggrandizing overstatement. Shelby does not know a lot of about anything. The problem is he also does not seem to know much about anything at all other than how to get women above his station to desire him for reasons that remain a secret to everybody but themselves.
"I must say, for a charming, intelligent girl, you certainly surrounded yourself with a remarkable collection of dopes."
Although there is a refreshing lack of irony here—the detective is right on target—there is irony in the context. The fact that Laura Hunt can be said to do this is indicative of the possibility that the entire last half of the movie is just a will-to-power dream of McPherson after he falls asleep in front of Laura’s huge portrait just before she supposedly comes back from the dead by walking through the front door.
"I don't use a pen. I write with a goose quill dipped in venom."
Waldo’s own description of his work as a columnist is positively Wildean in nature. It is also the most memorable line of the movie. Alas, the fact that Lydecker says it about himself says more about him than the actual words themselves.
"The best part of myself - that's what you are."
Tough to say whether this is intended to be ironic commentary or not. On the one hand, it is very easy to distrust anything that Waldo says which seems sincere in the slightest. On the other hand, Waldo must by the point he says this to Laura recognize that there really is no good part of himself that is actually a part of himself. It's a tough call.
"Goodbye, Laura. Goodbye, my love."
The last lines of the film are delivered to Laura from a dying Waldo, bringing the irony of the opening full circle.
