The writer is almost always describing a world or events that are not exactly like what we are used to in the physical world. Whether it is a hero who dashes through the scene with guns blazing on all sides without even minor flesh wound, a private eye who is able to out think both the police and the villain or the magical transportation of an historic figure to modern day, the reader, listener or watcher must be temporarily willing to accept these things and suspend his normal tendency to disbelieve.
This is the actual craft of the writer. You can have excellent dialog, description of characters, etc., but if the audience is not willing to suspend their disbelief and enter the universe that you offer him, he is not going to enjoy what you have to offer. While some writers and performers assume that the suspension of disbelief is the burden of the audience, I tend to think otherwise. It is the burden of the artist and is in fact his stock and trade. If he engages his audience they will suspend disbelief and forgive numerous errors in order to enjoy what you are presenting to them. In short they will enter your universe.
How does one achieve this?
The first thing that you have to do is to engage the audience. Give them something that they can agree with, some one to care about and sprinkle in a bit of suspense. If done well the audience will want to be part of your universe and see the characters through to the end of their story.
You should know what is real and what is not in your created universe. It should be consistent within itself. Otherwise you could confuse your audience and the are no longer willing to suspend their disbelief. If you are dealing in a world that is pretty much like our own, then you must have your technical details and facts down pretty well. Research out your technical details and have a knowledgeable person look over anything that you are not totally sure of.
In my own case, someone who had been trained in martial arts and weaponry read something that I had written and he noticed a tactical error that in his mind was unforgivable. Even though the back story had plenty of reason for this and the individual making the tactical error was not trained in that aspect of martial arts, it was still more than he could stand and he could not enjoy the rest of the work. Another person not trained in martial arts read the same material and enjoyed it despite a number of fantastic elements.
Another experience was when reshaping part of a story, I suddenly had evil characters who were just too evil for no reason. I needed to have some characters within the bad guys that could be identified with. Well, you can’t identify well with someone who is just blindly evil. So to suspend my own disbelief I redid the back story and gave the villains a real reason and a viewpoint of their own. They became not so much evil as they were trapped into a “rock and a hard place” situation that they could not escape from. So of course they were evil. From their point of view they were trying to survive just like everyone else.
All that said, if you run into someone who so completely refuses to suspend disbelief that he picks your stuff apart, turn a deaf ear and carry on. He was never your audience to begin with. Find the audience who can and will identify with your characters or other elements of your world and give them your story. If you really talk to them, they will suspend disbelief and enjoy.