Michael Schneider Variety Editor at LargeIf the idea of Kevin Hart as Arnold Jackson — the 8-year-old played by Gary Coleman on “Diff’rent Strokes’ — sounds perfect on paper, wait until you see tonight’s “Live in Front of a Studio Audience.” When the 5’4” Hart stands next to the 6’4” John Lithgow — who fills Conrad Bain’s role as Arnold’s adoptive father, Mr.