So there are two basic elements that can make or break a story. One is cohesiveness, the other truthfulness. The former is established through the universe the story takes place in, the latter through the characters inhabiting it.
Let's address the universe part first. What you are creating is a parallel universe, even when your intention is to have the story take place in 'this' reality, like when you set out to do a historical fiction or a crime thriller. The reason is, the world you create will be grounded in your experience and yours alone, and that is a pretty restricted view. However, it's your goal to share this universe of yours and have others have a shot at making it their own. To achieve this, others have to believe in it, and to have them believe in it, it has got to add up – there must be no pinpricks, that's to say.
Now the characters. Their task is to be your audience's eyes and ears in the universe you created. It's through them that other people can experience your universe. To achieve this, your characters have got to be relatable, which is a subjective thing: one character might do the trick for A, but not for B. You can't control your audience's reaction, but what you can do is control the degree of realness they are based on: pick a person from the real world, observe him, drop him into your universe. If done right, it's the characters that will dictate the story's plot; you just channel them (at least that's how I imagine it would be). If the characters are real, then so will their actions and reactions, and the audience will perceive this as truthfulness. One contemporary writer who is a master at this is George Pelecanos.
