There is little room for Deus ex machina in your writing. Contemporary readers will not stand for it. Things can not just happen: there must be motivation and the normal rules of physics must be obeyed. If you think, “Well that would be cool!” your next step is not to write it, but to research it: “Can this happen?” and “How could this happen?” and even “Are there people who know this can happen and can help me figure out how?” And that’s where, in the end, we get subtlety in fiction.
I’ll give you a for-instance: in one of my books, I had the heroine stuck in a tomb. And she nearly suffocates before she manages to get out. (It’s one of my favorite scenes ever: I could barely breathe myself when I wrote it!)
First my partner and I did the math: this much square footage + this much height = this much air. So how long would she have? It was too much time, so I wrote the tomb smaller. Then I contacted a physicist at UBC. A rocket scientist. And I got him to help me work out the details. And out of that process, I got some fantastic real life details I would never have had if I’d just made it up out of air. So after all that research, when my editor came to me and said, “I don’t think it would be this way.” I could quite easily say: “Yes. Yes it would. That’s exactly how it would be,” because I had already done all the miles.
If you’ve only gotten as far as, “Hey, that would be cool,” and you’ve written it thus, you’ve not gone far enough. Modern fiction is not made out of that. Or rather, it is much more sophisticated than that. Readers are smart and savvy and way demanding. There can be no element of your book that is not perfectly plausible. Not sure if it’s plausible? Go ask someone. Believe me: someone knows. We are writing in a world where a book that is good enough is not enough. It must be the best book it can be.