Think about when you tell the truth. The circumstances under which you wouldn’t bother to polish it up even a little bit.
Think about when you lie. The reasons you lie. The person you’re lying to.
You can ramp up your dialogue by finding the relative drama on a sliding scale of truth in a given situation. Is there drama in it? Is it just plain old, no stakes, why-bother-tweaking-it-truth? Then it’s wasting space because it’s not dramatic.
When you write dialogue, always consider the difference between what the character thinks and what they say. This will develop your subtext and make your script and your characters much more interesting.
Pro tip: Telling the truth and confessing the truth are not the same.