Man. So this is another of those hard questions I’ve been sitting on for days while trying to figure out my own answer. Let me answer the second question first, because I think it’s a little easier.
Short answer yes, always. Characterization is a fundamental tenet of fanfiction by its very nature; because we are writing about someone else’s characters in someone else’s world, it becomes doubly important to realize that readers often will have the same understanding of a character as the writer. It’s a heck of a lot harder to slip careless characterization by someone who’s spent just as many hours reading and writing that character as you have.
I do think characterization is something that becomes easier and easier the more time you spend in that character’s head. I’m sure there are plenty of people in the DA fandom who disagree with parts of my Fenris, but if nothing else I feel comfortable in my iteration of him and in his interactions with my Hawke. The more I’ve written him the more consistent he’s become, and I think consistency is really important in someone else’s characters. A Fenris who ends up with Isabela isn’t going to be quite the same as one who ends up with Sebastian as one who ends up with Hawke; but as long as the different shades cover same root we see in canon, you can take the character almost anywhere and your readers will still trust you.
All the same, there are times when no matter how much you’ve written for somebody, you’ll still hit a wrong note. Even in this most recent post-game fic I hit a scene with a particular character that felt off even as I was writing it, and sure enough, Jade pointed out that particular scene as ringing false in her beta notes. She was right, of course; I was forcing an emotional note for a character who wasn’t given to that sort of emotion, and no matter how much I wanted that beat I had to step back and make it true to the character first, even if it meant losing a bit of the poignancy. (Though really, considering the scene read untrue with the forcing of that emotion, the sacrifice was worth it. It was just the letting-go of what I wanted wah that hurt the most.)
So. Characterization is incredibly important, yes, because when writing fanfic your readers will know if it’s off. But it gets so much easier the more you practice, and even if it’s always something you have to keep an eye on, it’ll get easier and easier to recognize the weak places.
Now for your other question.
I have a jumble of thought regarding the first part of the question and I’m just gonna spill them here.
As people, we avoid pain and seek pleasure. And the same note shines in “I hate writing, I love having written”. The writing process itself can be painful, that’s why we avoid it. We procrastinate, distract ourselves, or our yelling brain tells us that it’s worthless shit anyway.
It’s an avoidance tactic.
Why do we associate writing with pain?
Because it’s so personal to us. We show our inner fears and emotions, we lay ourselves bare in what we write. We are also super critical of what we write, we fear that it won’t be good enough, for arbitrary measures of “good enough”.
I’m not sure what the solution is here, if there even is a general one with how personal this is. Someone suggested to associate not-writing with pain, to think of the disappointment for not giving this idea room, for not communicating your thoughts. Another suggestion is to accept the painful process as given and rewarding yourself when you’re getting through it.
Maybe we should analyze why the process is painful? Is our writing environment unpleasant? Have we fallen into a group of super critical people who rip apart everything we do? Is there a story we avoid to write but it’s stopping us now to continue another? Does the keyboard make our wrists hurt? Is the screen hurting to look at? Are we putting too high expectations on ourselves? Are we demanding impossible brilliance from us all the time?
I think we should find a way to make writing a pleasant experience, so that the incentive for avoidance is not so high.
How? That’s probably different for everyone. My approach is to lower my expectations, to say “fuck it, it’s only fanfic” and accept that sometimes I write mediocre stuff. It happens, the world won’t end because of it.
Try to find the joy and pleasure and then write.