Some of you people have REALLY strong feelings about this and I don't get it.
Characters talking in a panel DON'T have to have their mouths open in the drawing!
I think most of us would agree that, normally, a panel represents a specific moment occurring in the time between the proceeding and following. The creators are picking the moment they think best portrays and moves the story along.
That moment doesn't have to include characters with their mouths frozen open... it really doesn't.