I used to be a total "pantser", I'd just come up with a bunch of characters, throw them into a scenario and then just write these long, rambly scenes where they all snark at each other and obstacles they overcame too easily because they were just so cool and badass.
Nowadays though, I'm all about character arcs and growth and hitting those key act breaks and having themes and stuff.
I tend to start with some vague idea, usually a character and/or setting idea, then I come up with "What's the thing that happens that throws their whole life off?" and I extrapolate what the theme is from that and how I can explore that through different characters. I really like to build casts based on what traits each character can bring out in others through contrast, so to me, a really good cast is one where every character contrasts every other main character in some interesting way, allowing me to pair up any two characters and get an interesting dynamic.
I tend to loosely follow the "Save the Cat!" beatsheet, while writing scenes organically. I then sort of fit them to the framework after. It's a very iterative process. I don't think in a linear way, so forcing myself to work like that is never good.