The working title was "Les onze vies de l'abbé Pierre" ("The Eleven Lives of Abbé Pierre" in English).
The production team worked a lot on how to make him look like Abbé Pierre and where to stop. Eventually, they chose to not go overboard and kept Benjamin Lavernhe's nose; Frédéric Tellier stated that sometimes, an actor looks the most like someone else when he's not trying to look like them, like Joaquin Phoenix in Johnny & June: Pasión y locura (2005).
It took a long time to write the movie, because anything writer/director Frédéric Tellier could get his hands on was more of less hagiographical, or legends written by Abbé Pierre or his colleagues. When Tellier met Laurent Desmard, Abbé Pierre's personal secretary and president of the Abbé Pierre Foundation, everything cleared up: he talked about his memories, about moments that were not in the official biography, he helped him understand how Abbé Pierre was thinking, and told him about his doubts, his failures, his origins.
Frédéric Tellier wanted to cast only one actor, who would be able to personify Abbé Pierre young and old, so someone who looks rather young, who could then be aged with makeup and prosthetics. For the auditions, the actors had to play the winter 54 and Palais des Congrès speeches, and Benjamin Lavernhe immediately stood out.
With Renaud Chassaing, his director of photography, Frédéric Tellier worked to find the look of the movie, and they ended up using Lensbaby lenses, which create a peculiar blur by changing the field of view.