We Didn't Ask So Don't Tell: Why or Why Not Reveal A Character's Back Story

Ah back story. It reveals a lot about a person, in fact, it reveals everything about a person. Motivation, personality, interests, even psychological or physical trauma all point in the direction of back story. Was it simple as daddy teaching you how to ride a bike or complicated like watching daddy teaching you how to ride a bike by pushing you into traffic? 

Either way back story is vital to what makes your character who they are and you don’t even need to reveal their back story. Strange isn’t it? But in all actuality, in your head your character  could have a traumatic back story but it may never meet the paper because your readers don’t need to know. The top reason not to give away your character's back story is that sometimes it can bring the plot to a screeching halt if it's done at an inappropriate time/was unnecessary and added nothing to the story. “But how will I know when to reveal the characters back story?” Never fear because I’ll tell you with three simple steps: 

Who: "Who", as the title implies, is who affected your character in the past that now affects them in the present. This could be something like a family/friend(s) dying, kindness of a stranger, or just generally a person or group "Who" affected this character's life. 

In the Harry Potter series, the title character had his parents killed by Lord Voldemort which later in his life became his arch enemy. Lord Voldemort had killed many people before Harry's parents and serves as the main antagonist for this series, which means a lot of other people hate him too. One reason you would need to know that Lord Voldemort personally affected Harry because since Harry's parents died that means he had to live with his horrible aunt and uncle and his upbringing and personality are probably drastically different than if his parents had lived. The more important reason for this is even though Harry is the hero and Lord Voldemort is the villain (Harry should want to oppose him because he's evil anyway), knowing that Voldemort killed Harry's parents personally drives Harry to defeat him and gives you more of a reason to root for him. 

What: This one is a bit less obvious as "Who" because it's not a person that affected your character it was an event that took place that influenced your character today. The main difference between "What" and "Who" is as different as if a character's mother died of cancer (who) or if she was murdered (what). Murder is a direct event causing her death while cancer can't be controlled and is more natural. "What" can be used a bit more freely because it directly affects the character and it can be a variety of things like "Sam is afraid of flying because he saw a plane crash". The "What" is the plane crash and the affect is Sam being afraid of flying.

In The Silence of the Lambs, FBI agent Clarice Starling must talk to serial killer Hannibal Lecter in order to catch another serial killer. Hannibal is extremely manipulative and only likes to give hints to Clarice after he learns more about her past despite being able to read her like a book due to his training as a psychiatrist. At one point we find out that Clarice ran away. Quoth the movie (http://www.imdb.com/title/tt0102926/quotes): 

Clarice Starling: [tears begin forming in her eyes] And one morning, I just ran away. 
Hannibal Lecter: No "just", Clarice. What set you off? You started at what time? 
Clarice Starling: Early, still dark. 
Hannibal Lecter: Then something woke you, didn't it? Was it a dream? What was it? 
Clarice Starling: I heard a strange noise. 
Hannibal Lecter: What was it? 
Clarice Starling: It was... screaming. Some kind of screaming, like a child's voice. 
Hannibal Lecter: What did you do? 
Clarice Starling: I went downstairs, outside. I crept up into the barn. I was so scared to look inside, but I had to. 
Hannibal Lecter: And what did you see, Clarice? What did you see? 
Clarice Starling: Lambs. The lambs were screaming. 
Hannibal Lecter: They were slaughtering the spring lambs? 
Clarice Starling: And they were screaming. 
Hannibal Lecter: And you ran away? 
Clarice Starling: No. First I tried to free them. I... I opened the gate to their pen, but they wouldn't run. They just stood there, confused. They wouldn't run. 
Hannibal Lecter: But you could and you did, didn't you? 
Clarice Starling: Yes. I took one lamb, and I ran away as fast as I could. 
Hannibal Lecter: Where were you going, Clarice? 
Clarice Starling: I don't know. I didn't have any food, any water and it was very cold, very cold. I thought, I thought if I could save just one, but... he was so heavy. So heavy. I didn't get more than a few miles when the sheriff's car picked me up. The rancher was so angry he sent me to live at the Lutheran orphanage in Bozeman. I never saw the ranch again. 
Hannibal Lecter: What became of your lamb, Clarice? 
Clarice Starling: They killed him. 

The "What" in this case would be the lambs and as you can see it wasn't the rancher or the sheriff that affected her directly it was the lambs being killed. Though this is cryptic in its origin and you'd need some more spoilers from the movie to grasp the full concept, the "What" in this case shows this scar on the character while also showing how manipulative Hannibal is by exposing this part of her life. 

Why: "Why" is the answer to "Who" and "What" and is reserved for those special occasions when you don't reveal the character's back story. "Why" is when either the rest of the characters in the story or even the very readers question the character's motivations but know there's a reason the answer isn't revealed. Is it because it's too traumatic? Is the reason so minuscule it's almost laughable? Whatever reason you come up with will satisfy your mind if it's never revealed. 

In the manga series Death Note, the story about a serial killer who kills criminals supernaturally to become a god of the "new world", the detective who pursues him is named L. L is calculative, intelligent, and will stop at nothing to defeat Kira, what the public dubbed the serial killer. L has an incredible sweet tooth and usually gives his opinions in percentages ("I am 90% sure). Why is he like that? You never find out because the story isn't about the detective and the less you know about this genius the more mystery that shrouds this character and the more involved you are in the plot.