Alex Deuyour as himself: A 14 (nearly 15) year-old British-American wizard and the brains of the group.

Daniel Radcliffe as Harry Potter: Alex's godbrother, friend, and second cousin, who is famous for surviving Voldemort's attack as an infant, and is now starting his fourth year at Hogwarts.

Rupert Grint as Ron Weasley: Alex's godbrother, friend, ad second cousin, and Alex and Harry's best friend who is a younger member of the Weasleys.

Emma Watson as Hermione Granger: Alex and Harry's other best friend who is the other brains of the group.

Kate Nguyen as herself: Alex's crush and best friend who he takes it as friends to the Yule Ball.

Matt Crocitto as himself: Harry's best friend and a Gryffindor half-blood.

Robbie Caltrone as Rubeus Hagrid: The Gamekeeper and Care of Magical Creatures teacher at Hogwarts.

Ralph Fiennes as Lord Voldemort: A dark wizard intent on conquering the Wizarding World and the leader of the Death Eaters.

Michael Gambon as Albus Dumbledore: The Headmaster of Hogwarts and one of the greatest wizards of all time.

Brendan Gleeson as Alastor "Mad-Eye" Moody: A famous ex-Auror appointed by Dumbledore as the new Defense Against the Dark Arts teacher at Hogwarts.

Jason Isaacs as Lucius Malfoy: Draco's father and a former Hogwarts pupil of Slytherin House. He is also a member of the Death Eaters.

Gary Oldman as Sirius Black: Alex and Harry's godfather and Alex's maternal uncle, who had escaped from Azkaban after being wrongly imprisoned for twelve years and is now a fugitive.

Alan Rickman as Severus Snape: The Potions teacher at Hogwarts and head of Slytherin, who favors Alex as his godfather and honorary uncle.

Maggie Smith as Minerva McGonagall: Deputy Headmistress of Hogwarts, the Transfiguration teacher at Hogwarts and head of Gryffindor.

Timothy Spall as Peter Pettigrew / Wormtail: The Death Eater who betrayed Harry's parents to Voldemort.

Several actors from the previous film reprise their roles in Goblet of Fire. James and Oliver Phelps play Fred and George Weasley, Ron's twin brothers, and Bonnie Wright as their sister Ginny, while Mark Williams plays their father, Arthur Weasley. Tom Felton portrays Lucius Malfoy's son Draco, Alex's first cousin and Harry's rival, while Jamie Waylett and Joshua Herdman appear as Crabbe and Goyle, Draco's minions. Matthew Lewis, Devon Murray and Alfred Enoch play Neville Longbottom, Seamus Finnigan and Dean Thomas respectively, three Gryffindor students in Harry's year. David Bradley appears as Argus Filch, Hogwarts' caretaker, and Warwick Davis returns as Professor Filius Flitwick. Shirley Henderson reprises her role as Moaning Myrtle, a Hogwarts ghost, and Robert Hardy returns as Cornelius Fudge, the Minister for Magic. Louis Doyle appears as Ernie MacMillan and Charlotte Skeoch as Hannah Abbott; Hufflepuff students.

Robert Pattinson appears as Cedric Diggory; a Hufflepuff student who is a Hogwarts champion. Jeff Rawle appears as Cedric's father Amos. David Tennant plays Barty Crouch Jr., a Death Eater, and Roger Lloyd-Pack portrays his father Barty Crouch Sr., head of the Department of International Magical Cooperation. Katie Leung appears as Cho Chang, a Ravenclaw student and Harry's love interest. Clémence Poésy plays Beauxbatons champion Fleur Delacour, while Stanislav Ianevski portrays Durmstrang champion and Quidditch star Viktor Krum. Miranda Richardson plays The Daily Prophet reporter Rita Skeeter. Predrag Bjelac appears as Igor Karkaroff, Headmaster of Durmstrang and a former Death Eater, while Frances de la Tour plays Olympe Maxime, Headmistress of Beauxbatons. Shefali Chowdhury and Afshan Azad play Parvati and Padma Patil, Harry and Ron's dates to the Yule Ball, respectively. Eric Sykes appears as Frank Bryce, the caretaker at the Riddle family house.