In Harry Potter and the Half-Blood Prince, Albus Dumbledore (Michael Gambon) asks Harry (Daniel Radcliffe) to obtain a very precious memory from Professor Horace Slughorn (Jim Broadbent) instead of using Legilimency or Veritaserum on the Potions teacher – here’s why. As the war against Voldemort (Ralph Fiennes) becomes an urgent matter, the Hogwarts headmaster puts more pressure on Harry to find and destroy the Dark Lord’s Horcruxes. Slughorn’s memory is a crucial means to this end, yet Dumbledore fails to use several established methods to extract it.
At the beginning of the sixth Harry Potter movie, Dumbledore has already been hunting for Voldemort’s Horcruxes, as Harry sees his shriveled hand (a result of Marvolo Gaunt’s ring’s curse). From before the start of the school year, Dumbledore tasks Harry with getting to know Slughorn, as he knows it will take some time before Harry can get close and “penetrate his defenses.” Harry must obtain a memory Slughorn had tampered with involving himself and Tom Riddle, as their conversation (which Slughorn deeply regrets having) can confirm Dumbledore’s suspicion that Voldemort sought to split his soul into seven pieces. With Tom Riddle’s diary and Marvolo Gaunt’s ring destroyed, Dumbledore wants to know how many more Horcruxes there are, and which objects they may be. But Slughorn won’t give up that information easily.
By the events of The Half-Blood Prince, Dumbledore has already tried to obtain Slughorn’s memory. When he asks Harry to get friendly with the Potions Master, he tells him Slughorn will be expecting Dumbledore using both Legilimency and Veritaserum to get his memory: “He is much more accomplished at Occlumency than poor Morfin Gaunt, and I would be astonished if he has not carried an antidote to Veritaserum with him ever since I coerced him into giving me this travesty of a recollection.” Dumbledore thus can’t extract the Horcrux memory from Professor Slughorn by force. And as Dumbledore would never use the Unforgivable Imperius Curse, the only option left for him is to have a student befriend him and lead him to willingly share his memory – as it turns out, Dumbledore had a good feeling about Harry (as Harry had “a good feeling about Hagrid’s”).
When Horace Slughorn resigned as Hogwarts’ Potions Master in 1981, he did so because he was afraid Dumbledore would discover he had once told Tom Riddle about Horcruxes. 1981 was the year Voldemort was presumed dead, so for the first time in years, Slughorn thought he didn’t have to rely on Hogwarts for its safety anymore. However, when Voldemort returned in 1995, Slughorn knew without a doubt that Tom Riddle had created Horcruxes. He went into hiding, expecting Voldemort to either recruit or kill him to keep his secret safe. But Dumbledore had known about the Horcruxes for a while, and by 1997, Slughorn knew he knew. Although reluctant to join Dumbledore at first (“Taking up a post at Hogwarts just now would be tantamount to declaring my public allegiance to the Order of the Phoenix!”) for safety reasons, Slughorn soon realizes he must pick a side, and perhaps this is what ultimately led to him willingly giving his memory to Harry in The Half-Blood Prince.
As always, Dumbledore’s plan was carefully considered: he knew Slughorn’s propensity to “collect” his most talented students (as he once mentored Tom Riddle), and Harry was a bright and famous student who would make the perfect addition to the Slug Club. The headmaster oversaw Slughorn’s memory retrieval while Dumbledore was orchestrating his own death, which goes to show how well prepared he was in the war against Voldemort. In the end, asking Harry to befriend Slughorn was a smart (and crucial) decision. After Dumbledore died, Slughorn cemented his allegiance to Harry and fought his former student Tom Riddle at the Battle of Hogwarts. Ultimately, Slughorn’s memory in Harry Potter and the Half-Blood Prince was a milestone in defeating the Dark Lord.