We, as people, have been faced with the same temptations as those characters of the Bible. One time, I remember having a conversation with a writing teacher, and she said that I couldn't write a Bible story set in a different time because people of 1850 California weren't the same as Biblical people. Times were different.
I'll admit, times were different. Rules were different. But that is setting. People don't essentially change.
We can identify with the characters of the Bible because their emotions are timeless-- How would you like to be the dull-eyed sister married to a man in love with your hot sister? Yeah, being Leah doesn't get any easier with time. If you put it in modern times, your husband is cheating with your sister. Think of the betrayal there, the emotions. They don't get easier.
I like to ask the question of my characters, what would make her act out of character? What could break her?
This weekend, I watched "Jude the Obscure" -- Thomas Hardy is my favorite writer, but if you don't know this story, it is pretty dark.
Jude Fawley (Folly) is a religious man of character -- who falls prey to the neighborhood harlot, then ends up living with another unconventional woman he loves, without the benefit of marriage. He does all of these things because he's a man of character who wants to do "the right thing."
For years, he and his false wife are chaste, though no one believes that and Jude knows it -- he doesn't care what people think, only that God knows his truth. Hardy makes us believe in the character of Jude and his slow demise is believable because of the small steps that lead one into sin -- even when done with the right intentions.
If you're writing your current book, what would make them do something totally against type? How do you get them there? A lot of times it's the way God gets us to a different place. Life breaks us apart, and we must build back up.
My current book I started with the feeling that so many young people have today. They don't want to be married because they've watched their parents divorce. Marriage scares them. I wanted to take that a step further. What if you grew up in an abusive household? What if your parents never did divorce, but your mother thought God hated divorce, so she kept you in dire circumstances? How does that shape your view of marriage? Your view of God?
Don't be afraid to tear down your character to show what they're made of. Break it down into emotions. What is her biggest fear? What would make it worse for her? Go...be cruel to your characters. They're made of more than you think.