Wow! What a broad question! 😀
My characters mostly evolve slowly over time, I think. The initial spark of creation can be almost anything, from an idle 'what if..?' thought, to an idea from an unrelated conversation, to a memorable person I pass in the street, to a story requirement that demands some sort of personification or agent to make it happen.
These embryonic characters percolate and grow as I mull them over, sometimes forming their own connections that drive new plot arcs, slowly becoming more complex and detailed as they develop. This can sometimes be over years! I don't very often use formal character development techniques (questionnaires and detailed templates etc.) and prefer to let the person's traits and foibles develop organically as the plot or their individual story demands. Sometimes they will serve a particular plot purpose, of course, in which case their main character aspect might be there from the start as a necessity.
I do find the 'back-story' method of development quite useful, though, as Lisa Cron describes in her book Story Genius. This is where you develop and write full scenes from specific formative points in your character's life before the story opens. These open up and and explore certain areas of the character's personality development, and allow you to bring a character to their initiating event with a detailed and complex set of fears, flaws, pre-conceptions and motivations already in place - as well as the events that created them. These scenes may not get actually used in the book itself, but if a space opens up to explore the character's previous life they're there, all ready to be incorporated; so much more useful than a dry 'character sheet'!
What I share with you is a very visual aspect to character creation. Like you, I often do character sketches very early on, even immediately after the first conception of the person. As the character grows in detail and complexity so do the drawings and paintings, and I usually end up with a gallery of portraits of at least the main characters.
I also do this with scenes and settings, too, but it's the character studies that I benefit from most, I think. There's sometimes an extraordinary, almost spooky, moment when the painting stops being a mere depiction and suddenly a real person opens their eyes and gazes back at you from the paper, or the screen, staring out into your world from theirs - as if they have an existence outside your imagination and you're merely their chronicler rather than their creator! I find this 'reality' really helps anchor the character (although obviously not precluding change and development as the story is told).
Here's my current 'cast'.