Rowling personally insisted that the cast be kept British. Susie Figgis was appointed as casting director, working with both Columbus and Rowling in auditioning the lead roles of Harry, Ron and Hermione. Open casting calls were held for the main three roles, with only British children being considered. The principal part of the auditions took place in three parts, with those auditioning having to read a page from Harry Potter and the Philosopher's Stone, then if called back, they had to improvise a scene of the students' arrival at Hogwarts, they were then given several pages from the script to read in front of Columbus. Scenes from Columbus' script for the 1985 film Young Sherlock Holmes were also used in auditions. On July 11, 2000 Figgis left production, complaining that Columbus did not consider any of the thousands of children they had auditioned "worthy".
On August 8, 2000, the virtually unknown British actors Daniel Radcliffe, Emma Watson and Rupert Grint were selected from thousands of auditioning children to play the roles of Harry Potter, Hermione Granger and Ron Weasley, respectively. Columbus had wanted Radcliffe for the role since he saw him the BBC's production of David Copperfield, before the open casting sessions had taken place, but had been told by Figgis that Radcliffe's protective parents would not allow their son to take the role. Columbus explained that his persistance in givn Radcliffe the role was responsible for Figgis' resignation. Radcliffe was asked to audition for the role in 2000 Heyman and Kloves meet him and his parents at a production of Stones in His Pockets in London. Heyman and Columbus sucessfully managed to convince Radcliffe's parents that their son would be protected, and they agreed to let him take the role.
Tim Roth was offered the role of Severus Snape but rejected it to work elsewhere so the role was given to Alan Rickman. At the same time as Roth's negotiations, Robbie Coltrane and Maggie Smith, Rowling's personal choices for the roles, were cast as Rubeus Hagrid and Minerva McGonagall respectively. Coltrane, who was already a fan of the books, prepared for the role by talking with Rowling about Hagrid's past and future. Richard Harris initially rejected the role of Albus Dumbledore, only to reverse his decision after he was forced to by his granddaughter. Rik Mayall was cast in the role of Peeves, having to shout his lines off camera during takes, but the scene ended up being cut from the film. American actress Rosie O'Donnell held talks with Columbus about taking the role of Molly Weasley, although the role was eventually given to Julie Walters. David Thewlis auditioned for the part of Quirrell but was passed over for Ian Hart, Thewlis would later be cast as Remus Lupin in Harry Potter and the Prisoner of Azkaban.