One thing I don't get.

In TV or film where you have animated characters who interact with live-action ones, you often notice that the eyeline of the real actor doesn't match with the animated person they are talking to.

Now, if animation is done in post, why can't they correct it so that the eyeline matches?

If I'm into animation (I'm not but my cousin is) I would totally fix that.

"Hey! Eyes up here!"