La Peau de chagrin
The character Eugène de Rastignac had appeared as an old man in Balzac's earlier philosophical fantasy novel "La Peau de chagrin". While writing the first draft of "Le Père Goriot", Balzac named the character "Massiac", but he decided to use the same character from "La Peau de chagrin". Other characters were changed in a similar fashion. It was his first structured use of recurring characters, a practice whose depth and rigor came to characterize his novels.