Let's take, for example, Makoto of Sailormoon, otherwise known as Sailor Jupiter. She's first seen in the series as beating up a trio who had intended to harm Usagi, the main character. You immediately think, wow. This gal is tough, cool and beats up guys for fun. I'll write about her! She's caught by a few youma (Japanese for monsters; more on lingo used later) in an alley all alone without her transformation pen. She beats them up, gets a few scratches, when the other scouts/senshi finally arrive, she brushes it off like it was nothing. The other scouts/senshi are so in awe of her, they throw out Usagi as leader and nominate Makoto on the spot, which she graciously accepts.
If you post something like that, at worst you'll get flamed, at least you'll be ignored. This piece is hypothetical, and I doubt anybody would be so stupid as to write this, but I'm just using this as an example.
First mistake; you apparently didn't watch the whole episode. Makoto, while aggressive at times, is extremely protective of Usagi and her friends, because she has had very few. She can be vulnerable when it comes to emotions. Second mistake; all the senshi carry their pens in a subspace pocket where they can reach it no matter where they are. No chance of that happening. Thirdly; youma would give any person, even a scout in normal form, a nice rip in the chest. Fourth; Makoto is not stupid, nor are the other scouts slow. She'd call for back-up immediately. Fifth; youma do not know the scout's secret identities, so the chances of them cornering one out of millions are slim. Sixth; Makoto has always been the most avid supporter of Usagi, no matter what. She has always stuck up for her, even when Rei was vying to get the position of leader.
These facts are available anywhere on the Web. Every single one of these mistakes could have been avoided with a little intuition and research. Convinced yet?
Basically, most character development stories depend on information on events that occured in the character's life. When starting the story keep in mind that you should have all your facts at hand. You shouldn't put in specific dates which are hard to find and turn the reader off like a History lesson. But try your best to find out names, places and so on. When these facts are unmentioned, then most writers make up their own or use others. Note: Please ask the writer's permission if using fictional characters made up in his or her story.
Always remember; character development is from your point of view, not someone else's. You must think of what you are giving the reader; a better understanding of the character. Get your pertinent facts but know that this is a work of fiction, not a souless collection of facts. Don't worry about explaining everything to the reader; chances are that if they're reading it, they are already very familiar with the anime.