Are made of, go on, put them through hell.
Stretch them and give them high enough bars
To overcome before they do well.
Give them virtues, also give them flaws.
Show them making a pig’s breakfast out
Of achieving their just, noble cause
Actions show what your tale’s all about.
The best characters are the ones that stick in the memory and all of those tend to have flaws you can identify with as well as being heroic. My hero, Sam Vimes from Terry Pratchett’s Discworld series, is a decent guy but doesn’t bother hiding his prejudices (he’s learning to live with the other species around him albeit somewhat grumpily). And I love the character precisely because he is flawed yet still fights evil, criminality and, if he really could have his way here, stupidity.
Know your favourite stories and your best books.
Enjoy the tales and work out the hooks.
What makes the various characters work
Whether they be hero, villain, jerk?
Know what you like so you can write well.
Then comes the hard bit – making your work sell!
It makes sense to read well. To enter the book industry, such an industry needs to exist and it can only do so if it is supported. By knowing what you like (and dislike) by reading around, you can incorporate “good” influences into your own writing. A writer who doesn’t read seems an oddity to me. It’s like having a top ranking tennis player who’s decided to not bother with Wimbledon.