The first and most obvious is body language. Bodies and faces speak volumes. It’s not easy putting that into a story, though, so the body language needs careful construction to convey the subtext.

Next would be the things left unsaid. Sometimes, it’s in the thoughts of the POV character, which makes it more accessible to the reader to see beyond the surface of the interactions. Sometimes, especially if it’s not a POV character, there needs to be a combination of other things to indicate how the subtext applies.

And then there’s employing the life experience of the reader. Putting in just enough so their minds make the connection without needing to be told.

1. Body Language

There are a gazillion books out there about body language, but, as a general rule (very general) we pick them up as children within our community.
And community makes all the difference. If I mention that people in my home town stood about 2m apart when having a casual conversation at the shop, you’d know it was a country town – and you would also assume it was a first world country.
Community has its own rules. A religious community has very specific understanding of certain types of body language, and they recognise someone new or uninitiated.
A military community has its own language, and a lot of it is body language. It’s in the way they walk, talk, and scan the world around them. They know immediately if someone is muscling in where they don’t belong. Wearing the medals on the wrong side, or with mismatching medals, or using the wrong words to name something. You will be busted.
Body language isn’t the same for each person. We all come from a family, our first community, and then grow through a variety of other communities as we branch out into our lives as individuals. All those body language lessons go with us, keep us safe if we’re alert to the con who tries to fit in and can’t.
In terms of storytelling, body language is good as long as the sense of the community of the character is also obvious. They can’t all be using the same body language, and it’s an opportunity for problems to arise when there are differences to explore, or cons to expose.

2. Words Unsaid

A special type of subtext that also needs the context of community for the understanding to make it beyond the page. People who know the character may offer some advice to help with understanding why the character does things, ‘Oh, he’s just mad ‘cos Jules got the promotion,’ when referring to behaviour that is at odds with the ‘normal’ for that character.
However, it needs depth. And breadth.
Depth to cover the internal reasons why a thing would be left unsaid, and yet understood. Breadth to show how it applies to the life of this character.
‘It’s great to be home,” Justin said as he stepped back from the squalling tribe of grandchildren at the door.
What is the subtext of that action? Those words?
No, you tell me. I know what I think, and if I wrote them in a story, I’m sure I’d put more context to the situation.
A POV character has thoughts that can be shared with the reader, and that helps to understand the things unsaid, but don’t let that stop other forms of things unsaid make their play. Knowing how a character normally reacts, and seeing a different reaction should raise the question in the mind of the reader, at least, and in the other characters, too. The context of a war or a battle of the sexes can bring so much to the things unsaid.
‘I’ll go make tea, shall I?’ In the context of a war setting will show you a character desperate to bring some level of normal to the world. In the context of a battle between spouses, it can be interpreted as a stop sign, or a release of tension, or even a white flag. Context is everything with this level of subtext, so make the most of it.
The unsaid words can be extremely powerful, and that’s because it also uses the following level of subtext, although more subtle.

3. Employing Reader Experience

How would you respond if you saw a photo of a person patting a big cat?
Yep, that’s my pic. I patted a cheetah.
But you don’t know what happened after the photos were taken. You don’t know about the thunderstorm, and how cheetahs respond with great speed and agility to fearful events.
Did you imagine something? What picture did your mind create there?
I haven’t said what happened, but you brought your life experience to the words and created a visual or aural moment.
That’s called bringing the reader onboard and using their life history, understanding, and experience to make the story journey much closer to their own life journey.

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Okay, I’ve been very brief. It is only an introduction to subtext – because I don’t know enough yet to go further (always learning).

Why don’t you tell me what you know of it, how you use it to bring something extra and yet unseen to your stories, your life.