First and foremost it is a tune, and a tune that naturally depicts the Duke's sexual recklessness. It emerges from his throat as though he has sung it all his life. (G and Mcfarren, 1871).
One must admire 'La donna e mobile' for this reason- it is a tune set inside a musical score and complying with the necessity of transcending its setting. In the spoken play a sudden song must have some effect. But in an opera which is already wholly vocal, how much more striking must this ditty be! Verdi has achieved it, and within a tuneful score the Duke's song sticks out a mile. It is the right tune in the right place and how cleverly Verdi has given it an orchestral conclusion, diminishing from full orchestra gradually down to oboe and clarinet and then bassoon. (Godefroy, V, 1976).
Verdi was right not to give the tenor the music of "La donna e mobile" until the last possible moment. It is a vulgar, catchy tune, and he did not want its effect its purpose in advance. It is also precisely the right kind of tune for that character to sing in that context, and to complain of it in terms of pure music is to misunderstand the art of opera. It is in character, and it is immediately identifiable, as it needs to be, when it is used again at the end of the opera, at the moment when Rigoletto thinks he has the Duke's body in a sack. .
Hard on this comes the Quartet, that illustrious, timeless masterpiece. It is triggered off by the entry of Maddalena, who takes the Duke's fany at once. 'Frank Walker's mammoth excavation into Verdi's private' life has said that he himself once knew a Maddalena. She was Barezzi's (Verdi's father) maid, and later caused a scandal by marrying him after the death of his wife. Though this was later than Rigoletto, one can imagine that Maddalena was already eye-catching at least to Barezzi. The Duke loses no time in addressing her in affectionate terms. His easy going melodic line is an accurate fit with his character.