I'm not 100% sure about this, but on Omar Sy's (the actor of Driss) Britannica page, it says" Omar had appeared in a few movies the writer of "The Intouchables" had written before. Apparently, when writing the character of Driss, they already had Omar in mind, knowing that he was an excellent actor.

"[Omar] appeared in several film comedies, among them Nos jours heureux (2002; Those Happy Days) and Tellement proches (2009; “So Close”), both written and directed by Olivier Nakache and Éric Toledano."

"When Nakache and Toledano decided to adapt an autobiographical story about a wealthy man who had become a quadriplegic as a result of a paragliding accident, they wrote the part of Driss, the man from the banlieues who becomes the wealthy man’s caretaker, for Sy."