The actresses only read the script once. Abdellatif Kechiche insisted that they forget what the script said line for line, and instead asked them to improvise their scenes and really let their actions and words come out naturally and as unforced as possible.
There was controversy surrounding Abdellatif Kechiche and his work methods. It was revealed that he would do hundreds of takes for small scenes to achieve the desired realism of the story, and his intense directorial style was borderline abusive. Both actresses stated that the film looks so real because Kechiche pushed them to their breaking point, and that they were really struggling. They also said that they did not wish to work with him again. Stanley Kubrick has been accused of similar abusive working styles.
There was no makeup artist and hairstylist on the set. The actresses were not allowed to wear makeup for most part of the movie.
For the first time ever the Palme d'Or at Cannes was also officially awarded to two of the actors. Usually only the director gets the distinction.
A good portion of the B-roll footage with Adèle Exarchopoulos that ended up being used are in fact of the actress when she was out of character. The camera would be on her when she ate and even when she slept on the train while they were on their way to set. Since during this footage people kept calling Exarchopoulos by her first name instead of Clementine (the original name from the graphic novel), they agreed to have the main character's name changed to Adèle.