For me, preparing for a new save takes a lot of time! Gladly, I've been playing with the same save for over a year now and I'm not planning on letting it go for at least two more! :)
Before beginning to play, I always limit myself with a movie or tv show on which I base my game. I do that to stop my constant urge to write down my silly little sims worlds and turn them into actual novels (because for a writer that stuff can be very dangerous and damaging, trust me :D). My first save was a DC save, then I had a Whedonverse save, currently I'm playing in a Marvel save.
Right now I'm playing with Peter (aka Spiderman), the adopted son of Tony and Steve (Iron Man and Captain America) who has a sister Morgan (Tony's and Pepper's daughter), used to have a pitbull Tessa (because Tom Holland, the current actor playing Spiderman, has one) and is currently in love with Gwen (Gwen Stacy). He also used to do ballet (because Tom Holland did), modelling (again, Tom Holland), wears Tony's glasses (reference to the latest Spiderman movie), keeps an Iron Man action figure in his bedroom, etc, etc, etc. As you can see, references everywhere. I love references :) Most of them were prepared and written down before I actually began playing.
I don't really try to grasp the same looks of the characters because I'm terrible at creating sims based off real people but I still try to keep some features like the hair color, eye color, etc, so that takes a lot of time too. Their personalities usually start off the same as in the movies, but later evolve and change into something completely different. These kind of limitations make the game more challenging, fun and also help me keep my inner writer in tact because otherwise I would be stressing over how to turn everything I came up with in an actual book and the game would become a chore. But this way, it's like I'm writing fanfiction, which is nice and relaxing and doesn't require me being good or original or stressing over plot holes which there are A TON of :tongue:
The funniest thing is, all of those hard and long preparations are usually just the surroundings for the story. The limits - who gets to marry who, how many kids should they have, what will be their interests. Character and world building, basically. But as for the storyline itself, I usually just come up with something very simple and then let the game throw some curveballs at me to help it evolve. Because the game can come up with the most amazing plot twists ever, trust me! The main storyline of my current sims family happened because of something my game itself decided to do. It's just more fun that way :)