• Chris Hemsworth as Thor: The crown prince of Asgard, based on the Norse mythological deity of the same name. Director Kenneth Branagh and Marvel Studios chief Kevin Feige chose Hemsworth after a back-and-forth process in which the 25-year-old actor was initially dropped from consideration and then given a second chance to read for the part. Hemsworth stated that he gained 20 pounds (9 kg) for the role by eating non-stop and revealed that "It wasn't until Thor that I started lifting weights, it was all pretty new to me." After taking an initial costume fitting, Hemsworth returned three weeks later to find it did not fit because of the mass he had cultivated in that time span. At that point, he shifted away from eating a lot of calories and training to a kettlebell workout that would maintain his muscle mass rather than continuing to add to it. Regarding his take on the character, Hemsworth said, "We just kept trying to humanize it all, and keep it very real. Look into all the research about the comic books that we could, but also bring it back to 'Who is this guy as a person, and what's his relationship with people in the individual scenes?'" About approaching Thor's fighting style, he remarked, "First, we looked at the comic books and the posturing, the way [Thor] moves and fights, and a lot of his power seems to be drawn up through the ground. We talked about boxers, you know, Mike Tyson, very low to the ground and big open chest and big shoulder swings and very sort of brutal but graceful at the same time, and then as we shot stuff things became easier." Dakota Goyo portrays a young Thor.
• Natalie Portman as Jane Foster: A scientist and Thor's love interest. Marvel Studios stated in an announcement that the character was updated from the comics' initial portrayal for the feature adaptation. When asked why she took the role, Portman replied, "I just thought it sounded like a weird idea because Kenneth Branagh's directing it, so I was just like, 'Kenneth Branagh doing Thor is super-weird, I've gotta do it.'" Portman stated that she really wanted to do a big effects film that emphasized character, and getting to do it with Branagh was a new way of approaching it, relative to Star Wars. Regarding her preparation for the role Portman remarked, "I signed on to do it before there was a script. And Ken, who's amazing, who is so incredible, was like, 'You can really help create this character'. I got to read all of these biographies of female scientists like Rosalind Franklin who actually discovered the DNA double helix but didn't get the credit for it. The struggles they had and the way that they thought – I was like, 'What a great opportunity, in a very big movie that is going to be seen by a lot of people, to have a woman as a scientist'. She's a very serious scientist. Because in the comic she's a nurse and now they made her an astrophysicist. Really, I know it sounds silly, but it is those little things that makes girls think it's possible. It doesn't give them a [role] model of 'Oh, I just have to dress cute in movies'".

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Sam Raimi originally envisioned the idea for Thor after making Darkman (1990); he met Thor co-creator Stan Lee and pitched the concept to 20th Century Fox, but they did not understand it. Thor was abandoned until April 1997, when Marvel Studios was beginning to expand rapidly. The film gained momentum after the success of X-Men (2000). The plan was for Thor to be made for television. UPN was in talks for airing it; excited by the prospect, they pushed for a script and approached Tyler Mane to play Thor. In May 2000, Marvel Studios brought Artisan Entertainment to help finance it as a film, but by June 2004 the project still had yet to be patronized by a studio. Sony Pictures Entertainment finally purchased the film rights, and in December 2004 David S. Goyer was in negotiations to write and direct. By 2005, though there were talks between Goyer and Marvel, Goyer was no longer interested, though at this point the film was still set to be distributed through Sony Pictures.

Mark Protosevich, a fan of the Thor comic book, agreed to write the script in April 2006, and the project moved to Paramount Pictures, after it acquired the rights from Sony. That year the film was announced to be a Marvel Studios production. In December 2007, Protosevich described his plans for it "to be like a superhero origin story, but not one about a human gaining super powers, but of a god realizing his true potential. It's the story of an Old Testament god who becomes a New Testament god". Protosevich's script saw Odin banishing Thor to Earth in the Middle Ages, where he becomes enslaved by Norsemen before being rescued by Lady Sif and the Warriors Three. In August 2007, Marvel Studios signed Matthew Vaughn to direct the film. Vaughn then rewrote Protosevich's script in order to bring down the budget to $150 million, as Protosevich's first draft would have cost $300 million to produce. After the success of Iron Man, Marvel Studios announced that they intended to release Thor on June 4, 2010, with Iron Man 2 being used to introduce the character of Thor.
Vaughn was released when his holding deal expired in May 2008, at which point Marvel set Protosevich to work on a new draft and began searching for a new director. Marvel Studios also approached J. Michael Straczynski to work on an outline. Guillermo del Toro entered talks to direct the film. Del Toro was a fan of Jack Kirby's work on the comics, and said that he loved the character of Loki, but wished to incorporate more of the original Norse mythology into the film, including a "really dingy Valhalla, [with] Vikings and mud". However, del Toro ultimately turned down Thor to direct The Hobbit. By September 2008, D. J. Caruso had been discussing taking on the project, though he did not read the script. Later that month, Kenneth Branagh entered into negotiations to direct, while Daniel Craig was offered the leading role, but ultimately turned it down, citing his commitments to the James Bond franchise.

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The film's score was written by composer Patrick Doyle, a frequent collaborator of Branagh. Doyle described Thor as "the most commercially high profile film I have done since Frankenstein", adding that the composing process had the challenge of trying to find a tone that fit the duality of Asgard and Earth. Thus Doyle and Branagh had frequent discussions on the musical direction, with the director suggesting a contemporary feel and having a balance between the music and "grand images [that] were not in any way hyperbolized", and the composer in turn implementing "a strong sense of melody, which he responds to in my work". As Doyle declared that his own Celtic background made him familiar with Norse mythology, an old Celtic folk song also provided the inspiration for Thor's leitmotif. A soundtrack album was released by Buena Vista Records in April 2011.

The film also features a song by the Foo Fighters, "Walk", in both a scene where a powerless Thor shares some boilermakers with Selvig in a roadhouse, and the film's closing credits. Marvel president Kevin Feige stated that "Walk" was a last minute addition, that the crew felt had "these eerie appropriate lyrics and themes" upon hearing it. Branagh in particular thought that "these lyrics about learning to walk again" were appropriate "of [a] movie about redemption, learning to be a hero."