There's an episode of Star Trek: Enterprise I think about sometimes. Without looking it up to double check the details, the scene is this: Jonathan Archer (the Captain) is going on a dangerous away mission and leaving his dog Porthos in the care of his chief engineer Commander Trip Tucker. The dialogue ends something like this;
"and every now and then, give him just a little bit of cheese."
Simple, right? Just a man explaining how to look after his pet.
Except it's not.
This line haunts me because it's the single best crafted line I think I've ever watched on TV. Which sounds ridiculous, because it's talking about cheese. But in the context of the rest of the show, this is so much more than just one line.
This is a long running show; this isn't the first time we've met these characters. We know Archer and Trip go back a long way and they're good friends. And it isn't the first time we've seen Trip look after Porthos, either. Usually when Archer is leaving him with Trip, he'll say something like "Remember - no cheese!" It's a running joke - Porthos loves, but isn't allowed, cheese.
Except that Archer does give Porthos cheese. He knows it's not good for him so he doesn't do it often, but because it's his favourite and he loves his dog, every now and then as a special treat he lets him have a piece of cheese. Again, it's a running joke - Archer acts strict but he's actually very indulgent when it comes to Porthos and cheese.
So, going back to that one line. Looking at it in the context of all these other facts that we've learned, this line is more than just a simple instruction. If we break down everything that Archer is actually saying, it would read something like;
"I really don't think I'm coming back this time. I love you. Thank you for being my friend and looking after my dog. I trust you to do the right thing for him. Please look after him and love him like I do."
Pretty different, huh? So why doesn't Archer just say all those things instead? Surely it'd have more dramatic impact?
Welcome to the joy of subtext!
One of the most common mistakes we can make as writers is treating our audience as if they're stupid. Readers/viewers don't need to have everything spelled out for them; on the contrary, they like being able to put the pieces together for themselves. Everyone loves a mystery, right?
By leaving all those things unsaid, the scene becomes more emotionally charged. Especially in TV, where you have an actor who can imbue those lines with personality for even more impact, things don't always need to be explicitly stated. By letting the audience infer what Archer really means, we're placed in the same position as Trip (who is also going on the subtext), creating an intimate moment instead of a dramatic, over the top one. The scene is more poignant because only the two of them (and us, the viewer) really understand what's being said.
This particular example works really well because it's got several seasons of canon to back it up and it can be hard to get that level of depth into a stand-alone novel. But subtext can work really well in a number of different ways in a book, or even a short story.
First, as a teaser. You don't need to give all the information at once - dropping some hints in the subtext will intrigue your reader and make them want to dive deeper. I recently rewrote the opening chapter of one of my fantasy WIPs when I realised I'd filled the first pages with world-building information that the reader didn't actually need that early. Instead, the book now opens by introducing the main characters, and through their dialogue and interactions we get subtle hints about the world they're living in. Then things are explained slightly further on in the book, when we're already invested in the characters and we're ready for the context. I'll say it again because it's worth repeating; readers aren't stupid. If you treat them like they are, you'll lose them.
Subtext can also be great during a re-read. If you have a big reveal or a twist in your book, drop little clues throughout in the subtext. Then when your readers come back for a second go, they'll be kicking themselves at all the bits they didn't pick up on the first time round!
Of course these are only a few examples of how you can make the most of your subtext - let me know if you've got any favourites in the comments!
But for now; no cheese.