I saw the movie Wanted this weekend. While there was plenty of action, there was one small detail that kept nagging at me, and still does on Monday morning:
The main characters are given the names of people to assassinate from a loom.
Yes, a loom - like the kind of loom that makes shirts.
This fact is not necessarily the end of all credibility. As long as there is a believeable explanation for it, it will work. It's kind of cool, actually.
However, Wanted offered no such explanation. No origin for this loom, no purpose, and most of all, how the followers of said loom figured out in the first place that the loom was giving them names, and also, how they knew that these names pertained to people that needed to be assassinated.
You're not going to tell me that it was just assumed, are you?
Anyway, the purpose of this post is to demonstrate how imperative it is that everything in your story has a purpose. There should never be assumptions.
Look at purpose as something that can penetrate the smallest details of your story. Imagine your main character's bedroom (if they're lucky enough to have one). Where is their bed? Where is their mirror? What posters are on the wall? If you place these items in your narrative, there should be a reason why each is placed where it is. This doesn't need to be outlined in the novel, but you - the writer - should know. Maybe their bed is placed next to the window because they like to see the moon at night. They're a fan of Led Zeppelin, so the posters on the wall are album covers. The mirror hangs on the door so that your main character can check their appearance before leaving for the day. All of this provides credibility, it feeds your character - their wants, needs, preferences.
Don't just have things in your story because they need to be there. Those things need to exist for a reason, and it's your job to provide that reason. Remember: never assume, or presume. Provide purpose.
Plot especially can give way to the greatest holes. In a narrative, one thing needs to give way to another. One event is caused by the one before - one purpose leads to another. If your main character has a purpose - a direction - in one scene, that purpose should lead them to the next scene, and then the next. That purpose/direction can change throughout, but it needs to lead to the next event. All of this provides credibility. The second a reader does not believe what you are telling them, that's when reluctance to turn the next page will set in.
Characters need to have reasons for doing what they do (a purpose), and each event in a story needs to have reasons for happening (a purpose). See how purpose feeds through the entire narrative?
If you are questioning a character or a scene, the necessity of either will be revealed when you question their purpose. If you can't find a reason for that character or scene to exist, then they probably don't belong in your narrative. Their existence can most likely be given to another scene or another character.