Sirius Black is an old friend of Harry Potter's father, James, a fugitive from Azkaban, and a member of the Order of the Phoenix. He is portrayed by Gary Oldman.
Sirius Black is an loving and protective godfather of Harry Potter, he befriended James Potter, Remus Lupin, and Peter Pettigrew in Hogwarts during their school years, and Sirius doesn't take a liking to his deranged cousin Bellatrix Lestrange. Since he was framed by Pettigrew for murder for which he didn't do, Sirius swore revenge on Pettigrew. Aside from his vengeance, his temper flares up at times, for instance, he assaulted the Fat Lady's portrait out of blind fury after being denied access to Gryffindor Tower without a password. Following the capture of Pettigrew, he and Lupin are persuaded by Harry to spare him, knowing that his father James didn't want to do to his friend.
Sirius Black is a slender, fair skinned man, with shoulder length dark brown hair, blue eyes, and short brown mustache and goatee.
Sirius Black is mentioned numerous times throughout the film; he is also seen in the newspaper explaining his escape from Azkaban.
Black escaped Azkaban and took refuge away from the Dementors. When Harry is running away from the Dursleys, Sirius in his dog form, spotted him, but flees shortly after the knight bus arrives.
On Halloween, Black broke into Hogwarts undetected to find Pettigrew, but after being refused entry into Gryffindor Tower by the Fat Lady without a password, Sirius lashed out by slashing her portrait before leaving to avoid being spotted. Following his assault, Dumbledore, Filch and the teachers increase security in the castle.
In the deleted scene, he is mentioned that Sir Cadogan (who is standing in for the Fat Lady, whilst her portrait is being repaired) had let Sirius in the tower, after he said the correct password from Neville's lost list of passwords.
Much later in the film, Sirius transformed in his dog form, charges at Ron and Scabbers, and drags them both into the base of the Whomping Willow (it is revealed that was actually going for Scabbers), leading to a passageway to the Shrieking Shack. Sirius is fully revealed in the flesh to Harry, Ron and Hermione. Harry overpowers Sirius, furious over his betrayal of his parents, but Lupin intervenes, and the two old friends reconcile. It is revealed that Sirius was actually innocent and that Pettigrew did betrayed Harry's parents. Snape arrives and attempts to hand both Lupin and Sirius to the Dementors, but Harry knocks him out, and the three blast their wands at Scabbers, transforming him into the human form, revealing to be Peter Pettigrew. Before Lupin and Sirius have the chance to kill Pettigrew, Harry stops them, and persuades them to spare Pettigrew, understanding that his father didn't want to murder any of his friends.
After exiting the Shrieking Shack and the Whomping Willow, Sirius hesitantly asked Harry if he would like to live with him, thinking Harry would not want to live with the aunt and uncle who had raised him; Harry enthusiastically agreed, thrilled at the idea of leaving the Dursleys and finally having a real home. However, things turn to worse as Lupin transforms into a werewolf at the full-moon, prompting Sirius to restrain him, but to no avail. This causes Sirius to fight him off in his dog form, unintentionally allowing Pettigrew to escape in his rat form. Sirius is badly injured by the were-wolf and collapses at the Back Lake. Harry finds him, but sees a swarm of Dementors coming, prompting Harry to fend them out with the Patronus Charm but to no avail. The Dementors nearly succeeded in sucking out Sirius' soul until a powerful Patronus resembling a stag warded them off. Harry and Sirius pass out shortly afterwards.
It is revealed that Sirius was captured, imprisoned at the topmost tower of the castle and sentenced to the Dementor's kiss, but Harry and Hermione went back in time to save him. Harry casts the Patronus charm to save his past self and Sirius from certain death. Harry, Hermione and Buckbeak arrive at the topmost tower to break Sirius from his prison. Sirius thanks them for releasing him from the tower and bids them farewell.
In the ending of the film, it is notified and mentioned that Sirius sent Harry the Fire-bolt broom (in replacement of his destroyed Nimbus 2000) along with one of Buckbeak's feathers.
Sirius Black has gone into hiding away from the Ministry of Magic and is only mentioned throughout the film. He does send a letter to Harry, but he didn't use Hedwig because all of the owls were identical; Sirius wants to talk with Harry in the Gryffindor Common Room at 1 O'clock.
Sirius can only talk with Harry via floo network, asking him if he put his name in the Goblet of Fire, and advises him to be more vigilant. Sirius also tells him that he has to participate in the Triwizard Tournament. Upon hearing Ron coming downstairs, Sirius disappears just before Ron asks Harry who he was talking to. After that, he isn't seen again for the remainder of the film.