Albus Dumbledore
Background
Albus Dumbledore is the headmaster at Hogwarts School of Witchcraft and Wizardry.
During the events of Harry Potter and the Sorcerer's Stone (2001) played by Richard Harris
Dumbledore uses his delluminator to capture the light from the streetlamps on Privet Drive. Professor McGonagall transforms from a cat and shortly after, Hagrid arrives on his flying motorcycle and hands Harry Potter to Dumbledore and he leaves a note to Aunt Petunia Dursley and Uncle Vernon Dursley. Ten years later, Dumbledore tells the first year students the rules including that the Dark Forest is off limits. The first to get sorted into their house is Hermione Granger, then Draco Malfoy, Susan Bones, Ron Weasley, and Harry. During Christmas break, Harry spends the a few nights sitting in front of the Mirror of Erised until Dumbledore catches him and warns him about the effects of the mirror. After retrieving the Sorcerer's Stone, Harry wakes up in the hospital to presents left to him by his admirers. Dumbledore tells Harry the Sorcerer's Stone has been destroyed and warns Harry that there are ways in which Voldemort may return.
During the events of Harry Potter and the Chamber of Secrets (2002) played by Richard Harris
Professor Snape threatens to kick Harry and Ron out of Hogwarts and Dumbledore and Professor McGonagall walk into Professor Snape's office and Professor McGonagall gives Harry and Ron detention. After Mrs Norris is petrified, Dumbledore and the professors walk up and Dumbledore tells the students except for Harry, Ron and Hermione to return to their rooms. Professor Snape says it is suspicious how Harry was present at the time and Gilderoy Lockhart tells him, Harry was with him. Dumbledore then mentions Professor Sprout's Mandrakes and tells Mr Filch, Mrs Norris will be fine. Later, Madam Pomfrey, Professor McGonagall and Dumbledore escort two other professors who bring Colin Creevey into the nurse's station on a stretcher in a petrified state. They take Colin's camera from his hands to look if he took a photo of his attacker, but the camera exhibits a poof of magic. Dumbledore tells Professor McGonagall the Chamber of Secrets has been opened again. Harry walks into Dumbledore's office and Fawkes erupts in flames and falls to ashes. Dumbledore tells Harry phoenixes bursts into flames when they die, but are then reborn from the ashes. Hagrid barges into the room and tells Dumbledore, Harry is not the guilty one. Dumbledore tells Harry he doesn't believe it was him that opened the Chamber of Secrets. Later, Hagrid lets Dumbledore and Cornelius Fudge come inside his hut. Cornelius tells Hagrid that he will have to take Hagrid to Azkaban due to the attacks on the students. Lucius Malfoy walks in and tells Dumbledore the governors have decide it is time for Dumbledore to step aside as Headmaster of Hogwarts. As Hagrid is leaving with Cornelius, he mentions to follow the spiders and someone will have to feed Fang. After they save Ginny Weasley, Dumbledore awards special awards to Ron and Harry and gives Ron papers to have Hagrid released from Azkaban. Harry tells Dumbledore he worries about the similarities between Tom and himself and Dumbledore tells him Voldemort transferred some of his powers to Harry when he tried to kill him. Lucius walks into the room with Dobby and is upset to see Dumbledore back at Hogwarts. Dumbledore shows Lucius the journal that was used to entrance Ginny and Dobby motions at Lucius to Harry. Before the end of the year feast, Dumbledore asks for a round of applause for Professor Sprout and Madam Pomfrey and all of the students and professors clap, including Madam Pince. Hagrid returns and everyone gives Hagrid a round of applause.
During the events of Harry Potter and the Prisoner of Azkaban (2004) played by Michael Gambon
Before their feast, Dumbledore introduces the students to Professor Lupin. Dumbledore announces that Hagrid will be teaching the Care of Magical Creatures course. Dumbledore then warns the students not to get in the way of the Dementors. Later, the Fat Lady in Painting disappears from the common room portrait. Dumbledore arrives with Mr Filch, who is holding Mrs Norris, and Dumbledore tells Mr Filch to round up the ghosts to search for the Fat Lady. Mr Filch spots the Fat Lady and she tells them Sirius Black is in the castle. At the Quidditch match, while Harry is chasing the snitch, he is attacked by Dementors and loses consciousness and begins to plummet to the ground. Dumbledore stops his fall. On the day of Buckbeak's execution, Dumbledore, Fudge and The Executioner walk to Hagrid's hut. They enter the hut and Dumbledore spends a lot of time talking and by the time they leave the hut, Buckbeak is gone. Dumbledore then tells Fudge there is no way anyone can accuse Hagrid of freeing Buckbeak due to him being with them the entire time. After Harry is found unconscious with Sirius, Dumbledore walks into the clinic and Hermione and Ron tell him that Sirius is innocent. He tells him that he believes them, but others probably wont. Dumbledore tells Hermione and Harry where Sirius is being held and gives hints to Hermione on how to save two lives. He leaves the clinic and sees Harry and Hermione and Harry and Hermione tells him they saved Sirius and Buckbeak and he acts like he doesn't know what she is talking about and leaves.