Where should our hero first be discovered? What's his sense of humour like? How does he react in a crisis? What would he order in a bar? Etc, etc...
Of course, the hard part with any character is keeping them consistent. Yes, they must change as the story progresses (or at least, some of them must). But their change has to suit their temperament, their moral compass, their personality.
A film I saw recently was a perfect example of how inconsistent characterisations can sink a story.
Country Strong is the "Star is Born" story of washed-up country superstar Kelly Cantor and her ill-fated comeback tour. Also on the tour are her lover, musician Beau Hutton and promising female singer Chiles Stanton. They are on the way up while Kelly's career has hit the skids because of her out of control drinking. But her manager husband James can't afford to let her stay in rehab and has bounced her out to put her back on the road.
Gwyneth Paltrow has a perfectly competent voice, but it is not the voice of a country megastar. Oddly enough, her husband is played by genuine country star Tim McGraw, who never gets to sing a note!
There is nothing particularly wrong with the performances in general and Leighton Meester, who plays Chiles, is an excellent singer who could easily be a real recording star.
The problem is with the way the characters are written. One minute James is a ruthless manager who is determined to use his wife until he destroys her. Next minute he's a good guy with a crazy drunk to contend with. Beau is romancing the vulnerable Kelly while trying to talk Chiles into bed - and he's meant to be the hero! Chiles is an idiot one minute and slyly clever the next. And as for Kelly herself, Gwyneth Paltrow is forced to run the gamut of emotions from A to way past Z. She's the most unpredictable and inconsistent movie heroine I've seen in a while.
The result is a bit of a mess, with the characters impossible to identify with because... who are these people? Do the actors even know? Does the writer?
Know your characters - and make sure they act like their natural true selves. Otherwise, you end up with a movie like Country Strong...