Well this is a very important, we should ALWAYS strive to seek and animate the subtext, and the subtext should ALWAYS be different to the text. This is so important that for non dialogue shots, Randy Cook would record the unspoken dialogue (subtext) of a shot for his animators to animate to in their scenes.
Here's an hypothetical scene - A girl appears from a fitting room and shows a new dress she's wearing to her boyfriend, she asks his opinion, he says "yeah, it's great".
If we have his body language mimicking that thought, (it's great) the animation could be fantastic, but the idea isn't very interesting. It's what we call on-the-nose acting, and is just that, acting the words without any deeper thought as to their meaning or what the character is thinking. If the boyfriend says "yeah, it's great", yet his body language says "I'm bored, can we go home now?" we'll have gotten inside the character and given him some depth and therefore created something much more interesting and entertaining.