Forgive me, but "It's fiction; who cares if it's realistic?" is the rallying cry of both bad writers and undiscerning readers.
The answer is (or should be) everyone. Everyone should care if fiction is realistic, or at least realistic within its own universe. It doesn't have to be realistic as in mundane, but if you create a fantasy/fictional world that operates on certain rules, you as the author need to follow those rules, or the experience becomes jarring.
To answer your questions about writing, characters should have a solid backstory. Even if you never show that background to the audience, even if no one else ever knows your character's reasons for acting the way he does, those reasons should be there in the back of your mind. If your plot is forcing your character to behave uncharacteristically, change the plot or change the character. If you've written a shy, loner kind of person, you should not have that character throw a huge party or go to the big football game just to get the other characters into the story. Figure something else out. If you've written an honest, forthright person, that person should not lie at every turn just to serve your plot. A well-written character invites the kind of debate Darcy inspires. No one is imagining that he is a real person, but we as readers are within our rights to question his motives.
Actors often go through this same process: thinking about their characters and how they react to certain things based on the character's history and personality. Here's what Colin Firth had to say about Darcy's first proposal:
"...Darcy is coming in with a very imprudent proposal, as he sees it. He’s saying to her, 'I’m going to put to you a proposal that may make me seem rash, irresponsible and even, possible, juvenile, but I don’t want you to believe I’m those things. I have thought through every detail of this; I know that my family will be angry, that people will frown on us and that our social positions are very different. So don’t think that I haven’t dealt with these issues–don’t imagine that I’m just some reckless schoolboy. Nevertheless, having thought it all through, I find that my love for you is so overwhelming that these objections are rendered insignificant.'
And from that point of view, it’s a terribly romantic proposal. I was a bit hurt when we filmed it, and everybody thought I was saying something terrible: I had got myself so far into the notion that he had come in with a really charming thing to say."
To sum up: it is not ridiculous for readers to dissect characters in this way. If the character can't stand up to the scrutiny, it's not a well-written character.
The answer is (or should be) everyone. Everyone should care if fiction is realistic, or at least realistic within its own universe. It doesn't have to be realistic as in mundane, but if you create a fantasy/fictional world that operates on certain rules, you as the author need to follow those rules, or the experience becomes jarring.
To answer your questions about writing, characters should have a solid backstory. Even if you never show that background to the audience, even if no one else ever knows your character's reasons for acting the way he does, those reasons should be there in the back of your mind. If your plot is forcing your character to behave uncharacteristically, change the plot or change the character. If you've written a shy, loner kind of person, you should not have that character throw a huge party or go to the big football game just to get the other characters into the story. Figure something else out. If you've written an honest, forthright person, that person should not lie at every turn just to serve your plot. A well-written character invites the kind of debate Darcy inspires. No one is imagining that he is a real person, but we as readers are within our rights to question his motives.
Actors often go through this same process: thinking about their characters and how they react to certain things based on the character's history and personality. Here's what Colin Firth had to say about Darcy's first proposal:
"...Darcy is coming in with a very imprudent proposal, as he sees it. He’s saying to her, 'I’m going to put to you a proposal that may make me seem rash, irresponsible and even, possible, juvenile, but I don’t want you to believe I’m those things. I have thought through every detail of this; I know that my family will be angry, that people will frown on us and that our social positions are very different. So don’t think that I haven’t dealt with these issues–don’t imagine that I’m just some reckless schoolboy. Nevertheless, having thought it all through, I find that my love for you is so overwhelming that these objections are rendered insignificant.'
And from that point of view, it’s a terribly romantic proposal. I was a bit hurt when we filmed it, and everybody thought I was saying something terrible: I had got myself so far into the notion that he had come in with a really charming thing to say."
To sum up: it is not ridiculous for readers to dissect characters in this way. If the character can't stand up to the scrutiny, it's not a well-written character.