I used to just get an idea and run with it, and then fizzle out around 30-50K. I could build a throne out of my unfinished novels.
Then I started researching writing, and I went to the other end of the spectrum, and plotting a novel out from stem to stern. That novel never got written because I felt like I had already written it.
Then I went for the middle ground and actually finished a few books.
Lately, I think it depends on the novel, not me. Some novels require a lot more preplanning than others. Like if I am setting the book on a fantasy planet, then I need to be able to see the world like it was as real as Earth.
My urban fantasy requires slightly different worldbuilding, because I am usually making up the culture of jinni or sidhe or something.
Mostly if the idea is going to hold my interest long enough to be written I need the following:
*a good feel for the main character. I might not know everything about them, but I need to be able to feel like I can walk around in his/her skin.
*a twist. Some aspect of the story that lights my mind on fire and makes me itch to write about it. It’s usually related to the main character and the magic I am using. My last book, it was exorcism, and the scientists doing experiments on souls in their secret lab a la Frankenstein.
*an idea of a few scenes that also has me itching to write them.