Suspension of Disbelief
In the world of fiction you are often required to believe a premise which you would never accept in the real world. Especially in genres such as fantasy and science fiction, things happen in the story which you would not believe if they were presented in a newspaper as fact. Even in more real-world genres such as action movies, the action routinely goes beyond the boundaries of what you think could really happen.
In order to enjoy such stories, the audience engages in a phenomenon known as "suspension of disbelief". This is a semi-conscious decision in which you put aside your disbelief and accept the premise as being real for the duration of the story.
Suspension of disbelief only works to a point. It is important that the story maintains its own form of believability and doesn't push the limits too far. There are many factors for the budding story-writer or film-maker to consider, including the following....
The initial premise can be quite outrageous as long as the story maintains consistency within that premise. There are many things about the Star Trek universe which are basically impossible in the real world, but because Star Trek makes an effort to work consistently within its own universe, the stories become believable. For example, as long as you're willing to accept that the Galaxy is mostly populated by humanoids then there is nothing within the series that will break the believability.
The quality of special effects must be believable. It is harder to suspend disbelief in movies where the special effects appear fake.
The genre will determine the lengths to which you can push believability. Audiences will be willing to believe an action hero can perform super-human feats, but the same feats performed suddenly in a romantic drama would result in confusion and disbelief.
Some stories purposely push the suspension of disbelief to the limit. The Indiana Jones movies were a good example, where the audience was expected to find the improbable antics amusing.
One important area of belief is in human actions and emotion. People must act, react and interact in ways which are believable. In cases where such interactions do require suspension of disbelief, the normal rules of consistency apply. Audiences are very unforgiving if they think a character is behaving in an unbelievable fashion.