There is an excellent biography of the composer, Tobias Picker on his web page. Therefore, the information will not be repeated here. Please see http://www.tobiaspicker.com.
Gene Scheer, the librettist also has his own site: http://www.genescheer.com.
The opera had its genesis one day when Tobias Picker's sister was dusting. She accidently knocked a book to the floor, picked it up and reread it. Realizing its potential as the story for an opera, she recommended it to her brother who was looking for a new subject.
In retelling the story, they made the characters more three-dimensional and softer. They also made it more romantic and less in the naturalistic vein. Unlike Zola's characters who have no free will, those in the opera can and do make choices. The words also had to be made more poetic to work with the music.
They eliminated the character of Olivier, changed Michaud's name to Olivier, and gave Suzanne a greater role. They also introduced a mad scene for Thérèse after Laurent throws her to the floor and goes to get the poison. Finally, they shortened the amount of time between Mme. Raquin's stroke and the end of the opera.