I usually start with ideas, and characters flesh out mostly from a need to come up with reasons for them to do the things they initially do. At that point it is all pretty "view from 50,000 feet":
What would I do if someone killed someone I love, and I knew who did it before the cops did? What would I do?
OK, so what would I do if I didn't hunt, and wasn't familiar with guns already?
What would the grief be like, especially knowing I had things to hide?
Would killing the first person change me? Would I continue? In the same manner?
What if I was caught? Would I attempt to flee, or even confront law enforcement?
The "bad guys..." why would they kill in the first place? Assuming more than one, would they actually all have the same reasons, or multiple rationales for their roles?
What would they do after, both to cope and/or reconcile who they are with what they did, and also if they knew I knew? What about if the cops started closing in?
I ask a lot of questions, and many of them require me to assign certain sort of 101-level traits to the characters. All before I start writing, when I am sort of making sure I have a feasible start, a feasible ending, and an ability to actually tie the 2 together.
By the time I start writing then, I have a fairly rough character but at least some idea of who they are and what makes them tick. The rest comes as I write, either in specific scenes, or as scenes flesh out, additional ideas or details come to me for later scenes.
I don't have much luck with extensive character sketches, interviews, etc., but that's me personally