Durbin's scheming vs Grayson's sweetness

Does anyone know how these two amazing singers got typecast in these roles in their youth? Was Durbin really the more mischievous and Grayson really the more angelic? Or was one cast in a certain way and the studio thought the other should be a contrast? I vastly prefer Durbin's devilishness but I do enjoy Gene Kelly's calling Grayson "Aunt Susan" in Anchors Aweigh. :)