A Day Like Any Other, Filled With Those Events That Alter and Illuminate Our Times

(Title is reference to old Walter Cronkite show, You Are There).
When coming up with reasons characters are the way they are, history is very useful. Not personal history (though obviously that’s important) but world history.
Take World War I. A lot of fictional characters came back from the war conscious they’d been in the most exciting event of their lives (if that seems a bizarre reaction to such horror, read Christopher Hedges’ War Is the Force That Gives Us Meaning for some perspective on this). Bulldog Drummond and Tommy and Tuppence (Agatha Christie’s Partners in Crime) turn to detective work to recapture some of that excitement. The Shadow began his war on crime out of a desire to continue fighting the good fight.
For a more recent example, Finch in CBS’s Person of Interest built his all-knowing supercomputer to spot potential terrorist plots in response to 9/11.
It doesn’t have to be war, either. Perry Mason’s secretary Della Street is introduced in the first book (Case of the Velvet Claws) as a woman from a good family who goes to work to support them after they lose their money in the Depression. Rex Stout’s female detective Dol Bonner and the Shadow’s aide Harry Vincent were similarly influenced. Marvel’s Firebrand was a radical leftist who became disillusioned with peaceful activism and turned terrorist.

Another way we can use history is as a marker about a character’s character. In Casablanca, for instance, Renault makes a reference to Rick having run guns to Ethiopia and to the Loyalists in Spain. I didn’t realize this when I first read it, but the audience back in the day would have understood that to mean Rick was solidly anti-fascist. You can do the same by having a character revealed as a former Freedom Rider or a Black Panther, part of the 1930s Bonus Army and so on. In Southern Comfort, my protagonist is an anti-Vietnam War activist turned terrorist who walks away from her group when they switch from blowing up buildings to blowing up people. I’m not sure how that will play out yet, but it gives me a good feel for her character.
For more on what shapes characters, here’s my Single Determining Incident discussion.