One of the many wonderful things is
the supporting cast - they aren't the most common actors - Francis Lederer was fantastic - what a handsome and so very Continental kind of guy! And he was NOT in other screwball comedies.
Rex O'Malley - who IS this guy? He's hysterical.
Hedda Hopper was not often seen - particularly in comedies-
and Mary Astor was better known for serious thrillers like The Maltese Falcon.
One of the reasons this succeeds so brilliantly (after the script, the leads, the setting, the gowns, the scenery and sets) is because they use superb actors whom we haven't seen in these kinds of roles before or since.