Marianne was silent; it was impossible for her to say what she did not feel, however trivial the occasion; and upon Elinor, therefore, the whole task of telling lies when politeness required it, always fell. She did her best when thus called on, by speaking of Lady Middleton with more warmth than she felt, though with far less than Miss Lucy. (Ch. 21)

Dear Elinor practices the art of “telling lies when politeness” requires it but is quite conscious she is bending the truth—as above. I think what politic lies are about makes a difference. Elinor’s speaking of Lady Middleton with more warmth than she felt is not on par with Willoughby misrepresenting his tastes, opinions and enthusiasms that represent, in part, what kind of man he is. Elinor cannot very well disagree with Lucy about her hostesses’ wonderfulness but I do not see such a rending of politeness if Willoughby disagrees with Marianne’s raptures in Ch. 10. In fact he does object (sometimes) only to charmingly submit to “the force of her arguments and the brightness of her eyes”. It seems an act of guile for Willoughby to purposely mislead Marianne about himself and whether he did it a little or a lot does not change what it is. If he was lying to be polite then why not just agree to everything immediately with less vigor as Elinor does with Lucy and be done with it? Instead by the end of the visit they “conversed with the familiarity of a long-established acquaintance” (Ch. 10). Politic politeness does not usually lead to intimacy and the familiarity of a long-established acquaintance especially over the course of a few hours but guile often can. (:D)