I'll start by answering the second bit of the question: what flaws would I shy away from?
Ignoring the kind of flaw that's really sociopathy - I mean, you couldn't introduce someone with "This is Crispin. He stamps on kittens but he has a lovely tenor singing voice"- I think the only flaw I'd find it hard to write would be if someone was boring. It's difficult to show boringness properly without actually just *being* boring.
(In real life, a crushing bore can be hilarious if you've got someone's eye to catch and you're both thinking the same thing; then you actively *want* the bore to keep going until you know what kind of replacement showerhead she had to get in the end, or whether the journey was longer or shorter taking the freeway; or how many touchdowns there were at the bottom of the ninth innings at the last Knicks game.)
Dandy Gilver (my 1920s detective character) has quite a dull husband, but I only ever show short snatches of him and it's more that he's excited about unexciting things than that he drones on.
Dandy herself has flaws and foibles that are more fun to write. I always get a kick out of people who don't have much self-awareness: snobs who think they're approachable; spoiled brats who think they're stoical; dictators who think they're solid members of the team. Dandy Gilver's total lack of insight about herself is one of her main flaws, but it's also pretty authentic for a character of her class at that time, when inward reflection would have been seen as feeble and shaming.
The other flaw that I think is central to Dandy Gilver - and probably saves her from being insufferable - is not at all authentic for a toff in the 20s. (This is the first time I've thought about it; I'm grateful to whoever asked this question and forced me to).
A key feature of very powerful people is their incredible self-confidence and, no matter how silly they look to us now, the British upper classes of the 20s and 3os had a great deal of power.
No doubt if Dandy had stayed in her own world - a pretty empty world of house parties and charity work - her confidence would have grown as she grew older and she'd have turned into a splendid old trout.
But she took up detecting and more is now asked of her than her upbringing equipped her for. She has to think on her feet and make it up as she goes along. She regularly gets it wrong too and so she has learned the value of caution and has even - unheard of! - learned to doubt herself sometimes.
I've always been clear that Dandy Gilver is not me, but we have the self-doubt in common. I'm right now having a minor panic because I've just discovered something I didn't know about her and I'm currently writing the first draft of her latest story. What if this knowledge make it impossible to write about her anymore without it being clunky? What if answering this question is the equivalent of looking down from the high wire? What if me realizing that she's making it up as she goes along stops me being able to the same?
Writing: it's not a short-cut to serenity.