Have you seen Ant-Man? It's a great movie: nothing deep but lots of fun. I think they call this a popcorn movie. While I enjoyed every moment of it, it was painfully formulaic. It felt like someone said, "Let's remake Iron Man with the cheek of Guardians of the Galaxy." If someone indeed said those words, well, they succeeded, and they did it very well.
(Skip this paragraph if you don't want some spoilers for the movie - the rest of the post is fine. The point made here is that concepts get introduced early on so that later developments have context.) My husband loved the movie, but one of the things he observed about it was that every single thing mentioned early in the movie was used later. For example, at one point, Hank Pym is explaining the powers of the suit, and you knew that each power was going to be used in some spectacular, in-your-face way, and they were. Pym's daughter complained to him that he wasn't allowing her to use the suit, and you knew that she would eventually get her own suit. Hank told Scott about how Janet died, by shrinking too small and entering the quantum realm, and you knew that Scott would eventually do that. There are many more examples; I just don't remember more of them because I've only seen the movie once. However, this happens a lot of other movies.
There's a reason for this, of course: if you don't introduce concepts, when they appear later, there's no context for them and may feel like deus ex machinas. Here's a Doctor Who example. In "Human Nature", the Doctor becomes John Smith by using a chameleon arch to turn him human. Later on, in "Utopia", we discover that Yana is the Master turned human by a chameleon arch. If we hadn't had the concept introduced earlier, we would have gone, "What? You expect us to believe that this little device can turn a Time Lord into a human?" Or else, "Utopia" would have had to include a large amount of exposition to make the arch concept believable.
Of course, an episodic TV show has a greater ability to prime the audience with new concepts for later use than movies do. In this particular case, "Human Nature" / "The Family of Blood" was written around the concept of the arch and stands alone as a story, so the object had a separate narrative use other than to explain how the Master managed to stay hidden all this time.
It becomes a lot more difficult when the concept has to be introduced in the same episode in which it will be subsequently used, and that's really where my problem lies. How do you introduce a concept so that the audience can accept its use later on without being completely obvious about it? I really don't want my readers to see something mentioned and think, "Oh, that's going to come up later," or have a problem be solved with the concept and the reader goes, "Of course they used that! So obvious!" As a corollary, how do you figure out if you're being too obvious?
A couple of examples...
In Neighbours, while the group of friends are chatting in the pub, the two newcomers, Will and David, mention that they're new to the town, and Ben and Amy tell them about a number of historical locations to tour. They happen to mention that part of the old church's grounds is closed off due to an archaeological dig, and David's interest in that starts him wondering if they would accept volunteer work, and then spurs him to volunteer later at a science center. Later on in the story, when Will is despairing of finding the alien artefact that will save the town, he realizes that the reason it only just now appeared on Earth was that it was always there, but hidden and only recently discovered at the archaeological dig.
In Mistaken Identity, which is a DW/Harry Potter crossover, Dumbledore mentions to the Doctor, when he's wondering why his sonic doesn't work, that the amount of magic in the air at Hogwarts prevents the technology from working. During the final magic battle, the Doctor neutralizes the enemy by boosting the sonic to shut down the enemy's spells, theorizing that since too much magic shuts down tech, too much tech should shut down magic.
In both cases, if the original mention wasn't made, the final solution would feel like I pulled it out of my butt. It's less of a problem in the second case, since crossover readers should know about the "magic interferes with technology" rule from the HP universe (or should have read "Hogwarts: A History"...), though then an alert reader would then wonder how the Doctor knew to do that. (There is a scene in the story in which the Doctor reads a whole bunch of books about magic, so he easily could have learned it there, but then there'd need to be a few lines of, "Oh yes, I read about that in a book" and it would still feel like being pulled out of nowhere and very, well, amateurish.)
(In the Neighbours example, I also wonder if the pub conversation feels too contrived to give an opportunity to mention the archaeological dig.)
So that's my problem. I enjoy introducing concepts that are used later, but I don't really know how to balance it so that things aren't completely predictable. How do you do this? How can you tell if you're being too obvious (or too obscure)?