[I]f I'd behaved sensibly, nothing more would have happened, everything else would have been nipped in the bud. (2.5)
K. believes that if he'd stuck to common sense (i.e., just ignored the court and all its weirdness), life would have quickly reverted back to normal. Instead, the court makes him self-conscious to the point where he questions everything that he does.
In [the written defense], he would offer a brief overview of his life, and for each event of any particular importance, explain why he had acted as he did, whether in his present judgment this course of action deserved approval or censure, and what reasons he could advance for the one or the other. (7.2)
Since K. doesn't know what he's on trial for, he's forced to defend everything he's ever done. This defense is actually an autobiography, where K. must interpret and justify his actions to the court. Writing the defense is also a process whereby K. can attempt to figure out what exactly he's on trial for.
In the end K., who was now simply glancing mechanically back and forth during the conversation, began to fall prey to his earlier fatigue and at one point to his horror caught himself, just in time fortunately, starting to rise absentmindedly, turn around, and leave. (9.2)
As K. gets better at interpreting the world of the court, he gets worse at interpreting everyday conversations, as he loses track of this particular conversation.
[I]t was not impossible that he might receive some form of decisive and acceptable advice from [the prison chaplain], something that might show him, for example, not how to influence the trial, but how to break out of it, how to get around it, how to live outside the trial. (9.13)
"How to live outside the trial" is pretty much the holy grail of the novel – fervently desired and hoped for, but never achieved. Instead, the prison chaplain gives K. some tools to cope with the trial, as we shall see in his attempt to teach K. how to read the parable of the Law.
"Don't be too hasty," said the priest, "don't accept another's person's opinion unthinkingly. I've told you the story word for word according to the text. It says nothing about deception." (9.16)
The priest gives K. some good advice when it comes to reading a text. K. is so absorbed in his own troubles that he doesn't concentrate on the text. The priest tells K. not to accept another person's opinion without carefully evaluating it first, and also to base his interpretation on the text itself, rather than just jumping to conclusions. That's pretty good advice for reading The Trial itself, by the way, and you shouldn't unthinkingly accept our humble attempts at explaining the novel either.
"The commentators tell us: the correct understanding of a matter and misunderstanding the matter are not mutually exclusive." (9.16)
How could that possibly be? Well, one way to look at it is this: the story is, yes, puzzling. There is no one way to understand the story; it seems to generate infinite possibilities for interpretations. We can never know if our interpretation is the "correct understanding" or a "misunderstanding" because there's always another interpretation out there that we haven't considered. Thus we can try our best to come up with the best interpretation of the story, and in fact we should try our best, but we should also be aware that our interpretation is only an interpretation and will always be limited as such.
"You mustn't pay too much attention to opinions. The text is immutable, and the opinions are often only an expression of despair over it." (9.16)
The priest certainly seems to contradicting himself. If we're not supposed to pay attention to opinions, why does he give us so many opinions? The priest only seems to give us interpretations in order to show how indifferent the text is to these interpretations. This seems to fit in with K.'s experience with the Law: even as he struggles to defend his innocence, to explain himself to the courts, the courts are indifferent to his efforts.
"No," said the priest, "you don't have to consider everything true, you just have to consider it necessary." "A depressing opinion," said K. "Lies are made into a universal system." (9.16)
This is probably the most direct statement the priest makes about the story. The story is not concerned with matters of truth, whether it is verifiable in the real world. (No, there isn't an actual gatekeeper somewhere out there, there isn't a gate before the law, there's no actual man from the country trying to gain admittance, etc. – these are all part of the story's fiction, and don't exist in the real world.) All the story really cares about is what is necessary, what makes sense, within its own boundaries – just like the court, which doesn't really care about the actual innocence of your average Joe, but only about maintaining its own system where everyone's guilty. Just like the novel, in fact, which has created its own fictional world and can't really be verifiable in the real world.
[K.] was too tired to take in all of the consequences of the story; they led him into unaccustomed areas of thought, toward abstract notions more suited for discussion by the officials of the court than by him. The simple tale had become shapeless; he wanted to shake off the thought of it. (9.17)
Perhaps that's what K. ought to do – shake off the story and the court. Perhaps all of these interpretations – and, by extension, his struggles with his trial – are just a way to keep him from living a normal life.
Logic is no doubt unshakable, but it can't withstand a person who wants to live. (10.9)
Sadly, K. is wrong. Logic can withstand a person who wants to live, as his execution demonstrates.