One way you could chart the trajectory of Long Day's Journey Into Night is to follow how willing the characters are to be honest with one another. As the play starts, everyone except for the eldest son is terrified of bringing up taboo subjects like drugs, alcohol, careers, and the past. When Jamie does bring them up (as he does so often), he's shot down by the rest of the family (twice with a blow to the face). Only by the end of the play, when the rules don't seem to matter any more, do the characters actually speak their minds.
The lies in this play aren't new, off-the-cuff, improvised lies; instead, they have been repeated often enough to be a comfort to the characters who cling to them.
The frequency with which body language and body types betrays the truth behind a bit of dialogue is evidence of a broader theme, throughout the play, of meaning residing primarily in organic, natural expression and environments.