Everybody has a background
When you're reading a story or book, have you ever come across a character who suddenly does something out of character? Perhaps they pick up a gun and shoot the weapon from the bad guy's hand, supposedly while panicked over their partner's death, and after refusing for 100 pages to even look at a gun, branding weapons 'barbaric and scary.' Adrenaline can accomplish a lot, but that behavior just doesn't make sense.
Characters – ones that have been carefully crafted, anyway – have a background. Just like real people, they had a childhood, people they loved and respected, various knocks and bruises that life has dealt them. So, there are two possible situations that could explain gun-shy Jill's sudden ability. The first - and far too often, the most likely – is that the writer did not create a real character. Hair color, eye color and a name do not – by themselves – make a character. So this writer tacked a few simple characteristics onto the name, like scorn for guns, and went on from there. When Jill found herself in this awful circumstance, the writer had her pick up the gun and shoot, and explained it all as 'adreneline'. That's lazy writing.
Or … Jill was taught to hunt and shoot by her beloved dad when she was just a little girl. She was good, a natural marksman, and she basked in her proud papa's approval. Then there was a horrible accident, and she accidentally shot her own father, killing him. She vowed to never hold a gun again. But when her partner – a man she secretely loves – is shot and apparently killed, she can't let that be unavenged, so she picks up his gun and shoots. The bad guy is lucky, because she wanted to kill, but her aim is rusty after all these years.
Yes, that could be the explanation, but a good writer won't spring it on the reader. There would have been some omens, some hints. First, that Jill feels more than friendship for her partner, but also that she knows more about guns than she's letting on. If those hints and omens weren't there, then the background might as well not exist. Adding a visit to Jill's mom after the shooting and having that mom explain it all just seems like the writer realizes he goofed, and so he threw together this explanation. More lazy writing.
Characters can be lazy. Writers can't be, not if they want to be good writers. They need to give their characters a previous life, an outlook on life that isn't quite like anybody else's. This is vital for main characters, becomes less important for co-workers, neighbors and others only marginally involved in the story being told. The need for a backstory pretty much peters out when you get to spear-chuckers.
Don't know what a spear-chucker is? Well, that's something to explore another day. See ya next week. Trudy