There is a certain quality to black-and-white films from the 1930s and earlier that makes you feel like you're watching a dream. It is probably the combination of picture quality - softer focus, some lack of sharpness, some flickering, some over- and underexposure as well as the lack of color - which leaves more room for the imagination than more modern films do. This quality fades during the 1940s and is completely absent from the 1950s onwards.
And there is perhaps no better example than Der Kongreß tanzt, with Lilian Harvey at her ethereal best as the Viennese glover who is arrested for an 'assassination' attempt - with a bouquet of flowers advertising her business - on the Russian Tzar, played by Willy Fritsch in his usual playboy element and Austrian movie staple Paul Hörbiger as Heurigen entertainer. Key musical elements are the core scene of the film, when Lilian is taken to 'her chateau' by coach to the strains of 'Das gibt's nur einmal' - easily one of the 1930s most popular songs and Viennese anthem 'Wien und der Wein'.