An animator called Dapoon asked me a question about subtext in animation on this post from a while ago. As my rambling answer expanded beyond the limitations of the comment box I decided to put it here.
Thanks for the question; it took me a while to get to grips with what subtext is to. Subtext is not adding extra actions to your animation but exploring the underlying meaning of spoken words and actions in the hope of creating a deeper performance.
We should always strive to seek and animate the subtext of what the character says and this can be in varying degrees of alignment to what the spoken words are. Sarcasm would be an extreme example; at this point the meaning of what the character is saying is the exact opposite of the words they're speaking. "It's cold in here" = "It's hot in here".
I like this Monty Python sketch where they play around with this subtext. The Drill Sergeant's obviously sarcastic tone and manner belies the fact he seems to be talking totally literally.
You could play this scene with the same text spoken with a literal tone and it would still make sense ... probably wouldn't be very funny though. The humour comes from our perception of what the Sergeant is asking, or not asking - the subtext.
Dapoon suggests a situation - A mother folding sheets, while she asks her son how his day at school was. Let's explore that.
For me an interesting area of subtext to explore would be in the mother's delivery. Let's say her words are simply "How was your day at school?"
She could be just asking in a way that suggests she always asks this and is not really interested, it's almost just a greeting, like a "Hello" in this instance the subtext is there but subtle. Where the subtext becomes more obvious is when she's asking with a hidden agenda to find out more about his day. She could say the same words but be asking -
"Did you get into trouble today?"
"How did you do in the test you had?"
"Did you speak to that girl you like?"
Or if she'd already had a call from the headmaster to tell her that the boy has misbehaved this could form the subtext of the delivery, it could be a rhetorical "How was your day at school?" = "I know what you've done at school"
As you can see the delivery and acting of these lines would be different but the words or text ("How was your day at school?") stay the same.
The folding of the sheets I would call a 'secondary action', this would be both separate from the performance but also linked to it, if she was annoyed at her son she would be folding differently from if she was just absently mindedly doing it while chatting to him.
Hope that helps!