I think I started abstract and then got more concrete. I like some character sheets- but the 100 questions challenges usually feel like a waste of time to me.. depending on what questions are asked '^^
Nowadays, after years with my characters, I've gotten to the point where I don't rely so much on character sheets or written descriptions. I know who they are and what makes them tick, because I've spent so much time with them- observing how they behave in different situations, how they interact with other people, and the choices they make.
But I definitely think this stuff (character sheets, descriptions, etc) is crucial in the early stages of creating your cast and story. At the starting point, you don't know your characters yet and likely only have a vague concept of who they are, so it's helpful to have a list of defining characteristics to guide you and help you decide how they'd behave in any given situation.
Whether you write it down or keep it in your head (which might be forgotten, if your project is a very long-term one like mine), I think it's important to know what their personality, motivations, fears, desires, relationships, strengths, and weaknesses are. Before any of the rest, you should know what role they're supposed to serve in the story in order to contribute to an overarching theme or plot point.
I guess this all doesn't absolutely need to be put down on paper. You just risk forgetting information or mixing it up in your head if you don't. Documents are easy to keep for reference, while memories aren't.
On the topic of describing your character in words... I'm of the opinion that you should be able to do this. One writing teacher I was following online brought up a fun and useful character writing exercise that stuck with me: Describe your character without mentioning their occupation, age, gender, appearance, or using the word "driven". If you struggle to do that, your character might need work.
If I were to do that exercise with my comic's main character, it'd look like this:
MC is a reserved, level-headed voice of reason and a natural leader. They are deeply compassionate and loyally devoted to their few loved ones above all else, but their fear of loss holds them back from building close connections with others and causes them to become unreasonably risk-adverse and prone to anxiety. These flaws hold them back from accomplishing their goals, but over the course of the story, they learn to accept that life, risk, and loss go hand-in-hand and gain a sense of courage, peace, and acceptance, allowing them to step up and fight for the causes they're passionate about.
Anyways, there's my two cents. These kinds of exercises are not only useful, but fun imo :]