(From a convo I had with someone on Tumblr; saving here for reference!)
I'm looking through your questions and some of them are interrelated... I'll answer them in chronological order for now, but if there's something that isn't clear or if you have more specific questions or would like more detailed answers, just let me know, okay? :) (Also, fair warning, sometimes my head kind of drifts and I veer off into left field when i'm trying to answer questions, so just reel me back in if I wander too far/lose my point.) ;)
Re: Organization: I still use notebooks and scraps of paper, too! Do you find yourself collecting notebooks just so you have fun/nice books to put your story notes in sometimes? I'm very bad for that when I see a cute notebook. ^^; Then they sometimse are too nice and I don't want to use them... hahaha
As for storylines/plots, I occasionally goof on a character's background when I'm actively working two fics at once, but usually I rely on my outline/notes and go back and forth between them to ensure I'm on track. Once I have the story in mind, I just go with it. The only time I've noticed myself crossing wires is actually within the same fic when the fic itself deals with something like time travel-THAT trips me up. But I learned my lesson (... after several false starts...) and build my outline pretty solidly to try and avoid crossing my timelines/events. Kind of like how police departments/detectives have murder boards, I have my plot boards. I have a few whiteboards (3' x 4') that I also track a few storylines on, so I make sure I have my idea cemented in place before I write it out. :) (I highly recommend pen & paper or the whiteboard, if you have 'em, for planning out long fics or multi-fic series! I have a really long piece of brown butcher paper for Forest Fire with sticky notes along the time line. Different coloured stickies for different themes/types of events.)
As for which idea would be best for which fic... I... just kind of run with it? Usually I have the idea in mind for the fic, and it'll mostly write itself. :) Though I also rely on my betas, for example
sariasprincy, if I'm trying to figure out how to fix a rough patch. When I find myself getting MANY MANY ideas, sometimes I'll wedge them into the fic I'm working on; other times I just start a new fic. (Going by my hideous number of unfinished WIP, you can see I have a bad case of squirrel brain, hahaha) Some ideas overlap, or I find myself repeating certain themes. It happens. Something just gets stuck in my head. ;) I bounce between fics I work on (and I beta for others), so there's always something story-related happening in the back of my head. I compartmentalize by chunking my time and setting up certain writing conditions: for example, door closed, headphones on, specific playlist I've made for a specific fic/scene/type of story, and then I review what I've written and where I want to go in my outline, and then writewritewrite. If that doesn't work, I flip through the other stories I am writing and wait for that 'ping' in my head that says, "THAT ONE!" and I settle in to work on the fic that I have the inspiration for. I wish I could be more regimented/structured in my work ethic, unfortunately I have other constraints on my time (kids + work). So I tend to grab what time I can when it's available. My time/life isn't able to be too structured, so my organization kind of has to be overly-structured so I can hop in/out of a fic with whatever bits of time I manage. (Which does affect pacing in my fics, I've noticed, but eh, we're only human.) :)
As for characters, hmmm.... I try not to write 'characters'. I try to write 'people'. (Does that make sense?... Hmmm) Sometimes a character's personality will be based on someone I know, or a portrayal of someone I've seen in a movie or met in real life. That helps me flesh them out a bit for things like body language, attitude, reactions, tone of voice, mental state, etc. Sakura, for example, has some similar traits in my fics, but certain aspects of her personality or mentality may be emphasized depending on the plot, setting, and action. For *most* of my stories, I have a good idea of how I want to portray the character, what growth or change I want them to achieve, and that helps me plan what I need them to go through or what I need to show to demonstrate/illustrate that change. In "Take My Picture", I had a very specific characterization/picture of Madara in my head: how he would speak, act, behave in different settings, etc. A reserved formality that comes undone with Sakura when they're intimate together, whereas he would *need* to be in control in other aspects of his professional/family life. But with her, he's free. :) For the most part, I'm pretty good at remembering the way I'm portraying characters, I think. I do notice I need to re-read previous chapters of a fic if I've put it down for a while and then pick it up again when there's a change happening to that character emotionally or in the plot; that's where I will compare the chapter to what I wrote previously and to the outline, to ensure that I'm not 'overdoing' the emotions or body language.
When something in a fic challenges me or stumps me, sometimes I give myself a break and do some 'warm up writing' to get back into the 'feel' of that particular chapter, character, scene, etc. By 'warm up writing' I mean I will write a scene with the same characters, but not the scene I'm actively working on. Maybe I will write an AU with those characters doing something different, or put them in a different setting, or throw them a curveball to see how they react in the scene. It helps me get my brain thinking about how each character/person would behave differently.
Kind of like stretching/loosening up my creative side before I work on the actual ('hard') chapter.
Anyway, those are a few things I do.
If you have suggestions for how to overcome writers' block, I am all ears. ;)
Or if you have playlists for music for setting different moods, please share! I LOVE finding and listening to playlists for different settings, atmospheres, moods, etc. :)