Alright, there are some pretty awesome character sheets out there. I don't personally use them, but I've seen others make them work. I'm not here to dispute the fact that they've got some utility, but I am here to point out one big problem with relying on a list of traits—which is generally how character sheets present these things—to define your character.
(Note: This also applies to Mary Sue tests. 'Not a Mary Sue' just means your character isn't a perfect storm of coincidence. It has nothing to do with likeability or even believability. <insert dictator here> isn't a Mary Sue, either.)
I think this approach is, if it's the only approach you take to figuring out a character, a really bad idea.
Take a moment to think about your best friend. What's their favorite food? What are they good at, and what are things they do that make you Google good places to dump a body?
While you were coming up with answers to these totally not intrusive questions, did you really just pull the information from some mental dossier, or did you picture past situations where these things have come up? (Anyone who watches 'Supernatural' knows about Dean and pie.)
The people you know aren't a list of traits or neatly checked boxes on a sheet of paper. They're an aggregation of moments and scenes, things they've told you and things you've seen them do. (Especially seen. People grossly overestimate how much data words actually offer. Go outside, guys.)
Let's take a simple trait like 'outgoing.' Being outgoing could mean working in a field that involves a lot of face time, enjoying parties, talking to random people on the train. But a college professor probably doesn't spend weekends half-conscious in a puddle of vomit or make polite smiling faces at every pleb on the bus. A 26-year-old high school teacher, on the other hand, would totally go out more—but what about during finals week when she's got to get all those grades in? What about a high school teacher with war PTSD, or a college professor who's, like, super chill dude?
Traits on a page don't say much about behavior, which is what happens in scenes, also known as those things your readers will actually be reading. Reducing someone to a series of flaws/strengths/quirks overlooks the fact that people don't behave consistently.
Environmental factors FTW, not to mention state of mind at the time. Do you behave the same way at work, home, and when you're out with friends? Would your boss list the same strengths and weaknesses in your personality as your sibling or your best friend?
If you're going to use character sheets, don't let them be the be-all-end-all of how you create or develop your characters.
What am I supposed to do now that you've ruined everything?!
Seriously, you can still use character sheets to track information.
Visualize these traits. A lot of people write scenes for practice, which is a fantastic way of figuring out a character. I base all characters off aspects from my own real life and interactions. When that's impossible (e.g. murderers), I look at TV shows, movies, and case studies, in particular Oliver Sacks' work for the weirder stuff. I always start with a mental picture.
Mental pictures are good.
See a person within that demographic act out a situation, and then make it happen on paper. Pull and twist like taffy so you've got exaggerations in the right places, and bam, motherfucker! (As a cross between Emeril Lagasse and Samuel L. Jackson would say.)
When you're writing a character, you need to consider reasons that might pull them one way or the other, factors that will draw out certain traits and weaken others (chew that taffy). All actions result from some type of motivation, anything ranging from your money to your life to your sanity. (Motivation runs a wide gamut. My favorite motivation for characters is trying to escape the inevitable misery of the plot.)
I find that this method inevitably includes character development (not necessarily positive or progressive). The person you started with hasn't had those experiences during the plot to sway them one way or another. Once they have, they're going to behave differently...or the reader will see their actions differently because of new information.
The best way to make a character a convincing person is to have them be a person.