Suspension of disbelief is where someone accepts obviously untrue things in a story because he/she knows it's a story. An example would be Clark Kent. Everyone mocks how Superman just puts on glasses and no one recognizes him, but everyone accepts it anyway.
But is it a choice? Am I choosing to accept the untruths or am I accepting them subconsciously or on instinct without a choice in the matter? In my experience, it isn't a choice. If something has a compelling story and doesn't infringe on my areas of expertise, suspension of disbelief is natural, happens subconsciously, and happens whether I want it to or not. On the flip side, when something has a bad story, especially egregious breaks from reality, like someone breathing in space with no explanation, or treads upon areas I focus on, such as computers, politics, or philosophy, I can't accept it, no matter how hard I try.
Fundamentally, suspension of disbelief is a form of belief. When it is happening, we are truly believing it, despite what we know. Belief is not a choice. I am not an atheist in regards to the christian god because I choose not to believe, but because I don't find the arguments for it convincing. I don't believe the sky is blue because that's what I choose, but because that's what I see everyday. I don't suspend my disbelief, or fail to, because that's what I choose, but because the show succeeds, or fails, to convince me it's real. For this reason, when writing fiction, you must convince the audience what they're watching, or reading, is true, even if there is magic and elves and talking snakes, because, if you don't, the magic and elves and talking snakes will distract them. However, remember that you can't catch them all. I deal with computers a lot, so someone majorly messing up how computers work immediately loses my suspension of disbelief, while others' suspension of disbelief might be fine. Try and convince the most you can, and be happy with that.