People on screen very rarely finish a meal or drink because it would make the scene too long. They need to keep the action going.
Did you know, almost all the German officers (regulars) on Hogan's Heroes, were played by Jewish actors? When he was offered the Col. Klink role, Werner Klemperer only agreed to do it if the show's producers promised that Klink would never succeed in any of his schemes and that he would always wind up looking foolish.