The genesis of the opera began in 1953 when M. Valcarenghi approached Poulenc to commission a ballet for La Scala in Milan. When Poulenc found the proposed subject uninspiring, Valcarenghi suggested instead a screenplay by Bernanos based on the novella Die Letzte am Schafott (The Last on the Scaffold or, in one translation, Song at the Scaffold) by Gertrud von Le Fort. The novella derives from historical events at the monastery of Carmelite nuns in Compiègne, northern France, in the wake of the French Revolution, specifically in 1794 at the time of state seizure of the monastery's assets; it traces a fictional path from 1789 up to these events, when nuns of the Carmelite Order were guillotined.
Bernanos had been hired in 1947 to write dialogue for a movie version of the Le Fort work. When it was judged unsatisfactory for a film he turned it into a play which was produced in Paris in 1949. Poulenc saw the play, but it was on suggestion of his publishing firm Ricordi to turn the subject into an opera. The opera was composed between 1953 and 1956. The idea for the opera came at at time when Poulenc had recommitted himself to spirituality and (though openly gay) Roman Catholicism. Some, such as opera critic Alan Rich, also see Poulenc's concerns for the travails of post-World War II France as a subtext within the opera.
Some sources credit Emmet Lavery as librettist or co-librettist, but others only say, "With the permission of Emmet Lavery." The libretto is unusually deep in its psychological study of the contrasting characters of Mother Marie de l'Incarnation and Blanche de la Force.
Dialogues contributes to Poulenc's reputation as a composer especially of fine vocal music. The dialogues are largely set in recitative, with a melodic line that closely follows the text. The harmonies are lush, with the occasional wrenching twists that are characteristic of Poulenc's style. Poulenc's deep religious feelings are particularly evident in the gorgeous a cappella setting of Ave Maria in Act II, Scene II, and the Ave verum corpus in Act II, Scene IV. During the final tableau of the opera, which takes place in the Place de la Nation, the distinct sound of the guillotine's descending blade is heard repeatedly over the orchestra and the singing of the nuns, who are depleted one by one, until only Sister Constance remains.
Poulenc acknowledged his debt to Mussorgsky, Monteverdi, Verdi, and Debussy in the dedication of this opera but still felt apologetic about the opera's conservative harmonic language saying "You must forgive my Carmelites. It seems they can only sing tonal music." Opera historians, such as Anthony Tommasini, feel there is nothing to be ashamed of, saying the "subtle and intricate tonal langague is by turns hymnal and haunting. Though scored for a large orchestra, the instruments are often used in smaller groups selected for particular effects and colorings. The most distinctive element of the score, though, is its wonderfuly natural vocal writing, which capture the rhythms and lyrical flow of the libertto in eloquent music that hardly calls attention to itself yet lingers with you."
The opera has been widely praised, and opera historian Charles Osborne reflected on it saying "The inexorable dramatic movement of the work is impressive and, in the final scene in which the nuns walk in procession to the guillotine chanting the Salve regina, extremely moving. Poulenc also found an easy and effective style to which to carry forward without monotony the scenes of convent life.