Some interesting observations ( by you )
What context do you apply to " Empty " ?... in your mind, from your description ,is it lacking in content ?.. self expression ?.. lack of rhythmical interpretation ?,.... a look of boredom ? .
As to " lilt ", i understand why people use that appendage, and i believe , in their eyes, it,s how they are describing the combinations of.. Swing, Sway, and Rise and Fall .
It is not a true technical description, but maybe a very apt one ! .
heres a word that you should put into your dance vocabulary when dancing waltz... Langerous... I think this may be closer to what you are seeking . ( difficult to tell without a visual )