From chapter 10, a passage that has always set my teeth on edge:

"Brandon is just the kind of man," said Willoughby one day, when they were talking of him together, "whom every body speaks well of, and nobody cares about; whom all are delighted to see, and nobody remembers to talk to."

"That is exactly what I think of him," cried Marianne.

It's clever the way Austen can show in one line from Marianne how she goes from being a young lady of very decided opinions (wrongheaded, a good many of them, but decided nonetheless) to parroting whatever nonsense Willoughby chooses to spout. And if I had a parrot like that, I'd take it back to the pet store and demand a refund. ;-)

It's ironic that hardly two paragraphs earlier, we see this:

"Marianne began now to perceive that the desperation which had seized her at sixteen and a half, of ever seeing a man who could satisfy her ideas of perfection, had been rash and unjustifiable." [italics added] Did I say Austen was clever? Make that brilliant: such economy of language and a maximum of ironic foreshadowing, plus humour.

Yes, Marianne is definitely making herself agreeable to Willoughby by chiming in with whatever he says. But it's a bit chilling to think how short a time it's been since she was willing to listen respectfully to Brandon and afforded him some compassion. Willoughby is definitely a bad influence on her.