Secondary Belief (as defined by J.R.R. Tolkien) revolves around the world that you have created. If x is not true in our world, but is true in your fantasy world, then you have the obligation to define x and to do it early on. To thoroughly define it–no matter how fantastic the explanation, so long as it is equally thorough–is Secondary Belief. Secondary Belief is wonderful, and it’s what quality works of fantasy and sci-fi are built on.
However, failing to properly or thoroughly define the otherwise inexplicable leads to Suspension of Disbelief. (Granted, this term is also used to describe certain storytelling techniques, but for the purposes of this post, I’m essentially referring to plot holes.) I consider this lazy writing. If you want to cross genres, I’m all for it, but give me a reason why they fit together. If you want to add elements of astounding fantasy, please do, but don’t forget to bridge the gap so I can understand how it works. After all, if you’re not willing to put in the time to build out your own world, why should I invest the time to step into it?
And not only is it your responsibility as a writer to connect the span between my reality and yours, but it’s also your job to to do it soon. The longer you wait to define these differences, the more you are asking me as your reader to suspend disbelief. Even if your explanation simply labels the idea as mysterious until it can be properly explained later, at least you’re giving me something to go on for the time being. Failing to give sufficient reasoning for elements of secondary belief before the story advances very far is nearly as bad as failing to give a reason at all.
As a classic display of contrast between these two elements, let’s look at Superman. Superman is immediately defined by his backstory. Being an alien from another world, he can possess a number of powers along with a susceptibility to Kryptonite, and we can find this easy enough to assimilate since we know next to nothing about other life-supporting planets.
However, Superman has come to our world (or an approximation of it), and thus we expect those in our world to react to events in a manner consistent with our world. Thus, if his close friends fail to recognize him as Clark Kent, we have now crossed over into suspension of disbelief.
The only way to justify this discrepancy would be to establish a reason for it. For example, if humans on this version of Earth all had something akin to color-blindness, but it only affected their ability to recognize faces based on their immediate surroundings.
Of course, this would not only be a silly plot device, but it would also be immensely difficult to sustain. Any time Lois Lane changed her hair style, the writer would have to remember that Clark would fail to recognize her. Likewise, if someone were to wear casual clothes, then none of his/her coworkers would recognize them.
While these examples do happen to varying degrees in real life, those situations tend to be more of a double-take rather than a complete inability to recognize someone. If my coworker wears a black suit every day, then I spot him out in a Hawaiian t-shirt and shorts, I might not instantly recognize him, but it shouldn’t take me long.
Well, I hope this clears up the difference between Secondary Belief and Suspension of Disbelief for you and strengthens you as you set out on your writing startup. For me, I find it helpful to avoid getting bogged down by definitions. Instead, I just focus on explaining details and asking myself a lot of questions. That seems to do the trick.