"I assure you," said she, "I would not stand up without your dear sister for all the world; for if I did we should certainly be separated the whole evening." Catherine accepted this kindness with gratitude, and they continued as they were for three minutes longer, when Isabella, who had been talking to James on the other side of her, turned again to his sister and whispered, "My dearest creature, I am afraid I must leave you, your brother is so amazingly impatient to begin." (8.2)
There are lots of different kinds of lies in this text. Here Isabella exaggerates her desire to stay with Catherine, then promptly turns around and ditches her anyway, which turns her earlier "kind" sentiment into something hollow.
In this common-place chatter, which lasted some time, the original subject seemed entirely forgotten; and though Catherine was very well pleased to have it dropped for a while, she could not avoid a little suspicion at the total suspension of all Isabella's impatient desire to see Mr. Tilney. (8.21)
Isabella has a habit of saying things that sound very nice, but that she doesn't mean. Isabella might not be actively lying all the time, but her white lies, exaggerations, and omissions really start adding up.
Catherine listened with astonishment; she knew not how to reconcile two such very different accounts of the same thing, for she had not been brought up to understand the propensities of a rattle, nor to know to how many idle assertions and impudent falsehoods the excess of vanity will lead. (9.31)
John's "idle assertions" point to his carelessness with what he says – his ultimate agenda is to serve himself, not to say things that are clear or even true.
Catherine could not tell a falsehood even to please Isabella, but the latter was spared the misery of her friend's dissenting voice, by not waiting for her answer. Her own feelings entirely engrossed her. (9.33)
Catherine stands out here for her inability to lie to people. Catherine is shown to be more considerate of other people's feelings, while Isabella is only concerned with her own.
"We soon found out that our tastes were exactly alike in preferring the country to every other place; really, our opinions were so exactly the same, it was quite ridiculous! There was not a single point in which we differed." (10.3)
Isabella is clearly altering her opinions in order to match whatever James says here. Once again, Isabella is transforming herself into someone else in order to please another person and to ultimately get something she wants.
"How could you deceive me so, Mr. Thorpe? - How could you say that you saw them driving up the Lansdown-road? - I would not have had it happen so for the world [....] How could you say, you saw them driving out in a phaeton?" Thorpe defended himself very stoutly, declared he had never seen two men so much alike in his life, and would hardly give up the point of its having been Tilney himself. (11.53)
John Thorpe shows one way of dealing with getting caught telling a lie – complete denial. John won't even tell another lie in order to convince Catherine that he was mistaken. Instead, he sticks to his original lie, even as Catherine calls him on it. For more discussion of this scene, check out John Thorpe's "Character Analysis."
"This will not do," said Catherine; "I cannot submit to this. I must run after Miss Tilney directly and set her right."
Isabella, however, caught hold of one hand; Thorpe of the other; and remonstrances poured in from all three [....] When every thing was settled, when Miss Tilney herself said that Tuesday would suit her as well, it was quite ridiculous, quite absurd, to make any further objection.
"I do not care. Mr Thorpe had not business to invent any such message." (13.17-19)
What constitutes a lie is debated here. The Thorpes and James think that it's fine to tell a convenient white-lie, while Catherine objects to lying of any kind. She especially objects to John interfering and lying on her behalf.
To Catherine's simple feelings, this odd sort of reserve seemed neither kindly meant, nor consistently supported; and its unkindness she would hardly have forborn pointing out, had its inconsistency been less their friend; - but Anne and Maria soon set her heart at ease by the sagacity of their "I know what;" and the evening was spent in a sot of war of wit, a display of family ingenuity; on one side in the mystery of an affected secret, on the other of undefined discovery, all equally acute. (15.26)
The diction (or way of speaking) here really hammers home to true nature of the Thorpe family, who are really good at using wit and deceit. The Thorpes are inconsistent, unkind, and ingenuous, or good at inventing things.
What could be meant by such unsteady conduct, what her friend could be at, was beyond her comprehension. Isabella could not be aware of the pain she was inflicting, but it was a degree of willful thoughtlessness which Catherine could not but resent. (19.1)
This passage suggests that deceit and careless behavior can be a matter of inattention. Though Catherine's internal use of the world "willful" suggests that she is somewhat aware of the degree to which Isabella is aware of her callous, or mean, behavior.
Catherine trembled at the emphasis with which he spoke, and sat pale and breathless, in a most humble mood [...] and the General, recovering his politeness as he looked at her, spent the rest of his time in scolding his daughter, for so foolishly hurrying her fair friend, who was absolutely out of breath from haste, when there was not the least occasion for hurry in the world. (21.5-56)
It is interesting that politeness can be lost and then "recovered" here. For the General, politeness is used to conceal certain aspects of his personality and to deceive others into thinking he is something that he is not, even if what he actually is is rude and unpleasant.
But the inexplicability of the General's conduct dwelt much on her thoughts. That he was very particular in his eating, she had, by her own unassisted observation, already discovered; but why he should say one thing so positively, and mean another all the while, was most unaccountable! How were people, at that rate, to be understood? (26.15)
This is one of the best thematic statements in the whole book on how lying gets in the way of communication between people. Catherine doesn't understand how to translate people's meaning, nor does she understand why it is necessary to deliberately lie and expect people to know that the opposite is meant. Lying is actually inefficient.
Thorpe, most happy to be on speaking terms with a man of General Tilney's importance, had been joyfully and proudly communicative [...] his vanity induced him to represent the family as yet more wealthy than his vanity and avarice had made him believe them. With whomsoever he was, or was likely to be connected, his own consequence always required that theirs should be great, and as his intimacy with any acquaintance grew, so regularly grew their fortune. (30.12)
John lies for purely self-serving purposes here. His own pride and vanity cause him to exaggerate everything around him, including the wealth of his friends.