Interior monologue has a close relationship with "subtext," but it isn't the same thing. Subtext is literally "beneath the text." It's what the character MEANS, rather than what the character is saying out loud. Subtext is what you should always base your acting decisions on (rather than the actual lines). This is also related to what's known as "operative words," or the most enunciated and emphasized word or words in a delivery. A classic example I've heard used is the line "I love you." If you emphasize the word "I" when you deliver that line, what you are saying isn't just "I love you." No, now what you are truly saying is "He doesn't love you."
If you emphasize "you," on the other hand, now what you are truly saying is again different, and the subtext has become "I don't love her."
So, if the operative word can help you discover the subtext, that's nice to know, but what the heck is interior monologue?
Well, a monologue is someone speaking aloud, often to themselves. It's almost like thinking out loud. So "Interior" monologue is the same thing, but it's internal. It's silent. It's the thought process that we don't let past our lips.
Why is this helpful for animators? Well, because if our hardest job (and it IS our hardest job) is to create a believable feeling that our characters have an internal thought process, then figuring out the interior monologue of a scene gives us actual thoughts to key off of, and actual changes in though process to base our acting decisions on.
Let's say that in the scene, a man and a woman are arguing, and he's jealous of the way she's been flirting with a friend. Her line is "I love YOU," with the emphasis on "you." So we know that what she means is "I don't love him, I love you." When you are animating to that line, you could say that the monologue is "I love YOU," and the *interior monologue* is "I don't love HIM! How could even think that? Don't you even know me?"
Now, when you are working out the acting decisions, you can treat the interior monologue just like actual lines of dialogue, and you can time your head shakes, blinks, searching eye movements, etc - you can time all that stuff off of this imaginary line that isn't ever heard, but through your animation, we will FEEL it.
And *that* stuff is the meaty stuff that will bring your character to life.