- 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.[10] 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."[11] 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?'"[12] 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."[13] 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.[14] 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.'"[15] 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.[16] 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'".[17]
- Tom Hiddleston as Loki: Thor's adoptive brother and nemesis, based on the deity of the same name. Hiddleston was chosen after previously working with Branagh on Ivanov and Wallander.[18] Initially, Hiddleston auditioned to play Thor but Branagh decided his talent would be better harnessed playing Loki. Hiddleston stated that the character was "a comic book version of Edmund in King Lear, but nastier." Hiddleston had to keep a strict diet before the start of filming because Branagh wanted "Loki to have a lean and hungry look, like Cassius in Julius Caesar. Physically, he can't be posing as Thor".[19] Hiddleston looked at Peter O'Toole as inspiration for Loki as well, explaining, "Interestingly enough, [Kenneth Branagh] said to look at Peter O'Toole in two specific films, The Lion in Winter and Lawrence of Arabia. What's interesting about ... his performance [as King Henry] is you see how damaged he is. There's a rawness [to his performance]; it's almost as if he's living with a layer of skin peeled away. He's grandiose and teary and, in a moment, by turns hilarious and then terrifying. What we wanted was that emotional volatility. It's a different acting style, it's not quite the same thing, but it's fascinating to go back and watch an actor as great as O'Toole head for those great high hills".[20] Ted Allpress portrays a young Loki.
- Stellan Skarsgård as Erik Selvig: A scientist doing research in New Mexico who encounters Thor.[21] Skarsgård stated that he was not initially familiar with the comic book version of Thor.[22] As to why he took the part, Skarsgård remarked, I "chose Thor because of [director] Kenneth Branagh. The script was nice and we got to rehearse and talk to the writers and do some collaborating in the process to make it fit us. So I had a very happy time on it. What I always try to do is immediately do something I just haven't done so I get variation in my life. I've made about 90 films and if I did the same thing over and over again I would be bored by now. I try to pick different films, I go and do those big ones and having done that I can usually afford to go and do some really small obscure films and experiment a little".[23]
- Kat Dennings as Darcy Lewis: A political science major who is Jane Foster's intern.[24] Dennings described her character as Foster's "little helper gnome".[16] Dennings stated that her role was expanded during the rehearsal process.[25] Dennings explained, "She's kind of like a cute, clueless, little puppy or maybe a hamster. There wasn't much on the page for the Darcy role to begin with and I didn't even see a script before I took the job so I didn't really know who Darcy was at first. But she really evolved—she's so much fun now even. She's very Scooby-Doo if that makes sense. She's always three steps behind and reacting to what's happening with these great expressions ... She gets things wrong and doesn't care."[26]
- Clark Gregg as Phil Coulson: A S.H.I.E.L.D. agent. Gregg reprises his role from the Iron Man films.[27]
- Colm Feore as Laufey: King of the Frost Giants and Loki's biological father, based on the mythological being of the same name, who in myth was actually Loki's mother.[28] Feore stated it took five hours for his makeup to be applied.[29] About his character, Feore remarked, "I am the King of Frost Giants. And if you've seen any of the Frost Giants, you know that I am, of course, the Napoleon of Frost Giants. We've got some massive, fabulous guys who dwarf me and come in at around eight-and-a-half feet, nine feet. But, no. Can't you tell by the commanding presence? I am the boss".[28] He said the Shakespearean training he shared with Hopkins and director Branagh helped keep production moving briskly, saying that "during the breaks, Tony, myself and Ken would be talking in Shakespearean shorthand about what the characters were doing, what we thought they may be like, and how we could focus our attention more intelligently. These were discussions that took no more than a few minutes between takes, but they allowed Ken, Tony and [me] to understand each other instantly without Ken taking an hour away to explain to the actors exactly what was going on. So that was enormously helpful."[30]
- Ray Stevenson as Volstagg: A member of the Warriors Three, a group of three Asgardian adventurers who are among Thor's closest comrades, known for both his hearty appetite and wide girth.[31] Stevenson previously worked with Kenneth Branagh in the 1998 film The Theory of Flight, and with Marvel Studios as the titular character in Punisher: War Zone. Stevenson wore a fat suit for the role, stating, "I've tried the suit on, and what they've done is kind of sex him up: he's sort of slimmer but rounder.". Stevenson said, "He's got every bit of that Falstaffian verve and vigor, and a bit of a beer gut to suggest that enormous appetite, but he's not the sort of Weeble-shaped figure he is in the comics. He's Falstaff with muscles. I've got this amazing foam-injected undersuit that flexes with me."[32]
- Idris Elba as Heimdall: The all-seeing, all-hearing Asgardian sentry of the bifröst bridge, based on the mythological deity of the same name.[33] Elba said Branagh's involvement was a major incentive to take the role: "[Branagh] called me up personally and said, 'I know this isn't a big role, but I would really love to see you play it.' It's Kenneth Branagh. I was like, 'Definitely'".[34] About the role Elba remarked, "I did green screen for the first time! I wouldn't like to do a whole movie of green screen, though. You kind of forget the plot a little—like being in a Broadway play and doing it over and over and forgetting your line halfway through".[35] Elba stated he has made a four-picture commitment with Marvel Studios.[36] Elba's casting prompted a proposed boycott by the Council of Conservative Citizens and a debate amongst comic book fans, some insisting it was wrong for a black man to play a Nordic god. In response Elba called the debate "ridiculous".[37][38][39]
- Jaimie Alexander as Sif: A warrior and Thor's childhood friend, based on the mythological deity of the same name. Alexander was best known for her portrayal of Jessi XX on the ABC Family series Kyle XY.[40] Alexander said that she was familiar with Marvel Comics before having taken the part, having grown up with four brothers.[41] Alexander said the part required hours a day in the gym, though training is not unfamiliar to her, explaining she was one of few girls on her Colleyville, Texas, high-school wrestling team.[42] Alexander described her character as "one of the guys" and that, "She's a very talented, skilled warrior and can stand on her own against any villain in the film". About her relationship with Thor she stated, "She is very loyal to Thor and cares a lot about protecting him and protecting Asgard".[43]
- Rene Russo as Frigga: The wife of Odin, queen of Asgard, mother of Thor, and adoptive mother of Loki, based on the mythological deity of the same name.[44] Russo stated in March 2011 interview that she has signed on for possible sequels, although she stated that "who knows how many I'll do".[45]
- Anthony Hopkins as Odin: The ruler of Asgard, father of Thor, and adoptive father of Loki, based on the mythological deity of the same name.[46] In an interview Hopkins stated he knew nothing of the comic. About the film he said, "It's a superhero movie, but with a bit of Shakespeare thrown in".[47] Hopkins stated, "I'm very interested in that relationship between fathers and sons", and that, "My father's relationship with me was cold. He was a hot-blood character but to me, cold. When I was young, he expressed his disappointment because I was bad in school and all of that. He didn't mean any harm, but I felt I could never meet up to his expectations." Hopkins expressed that he found a personal resonance in the Odin role, saying, "He's a stern man. He's a man with purpose. I play the god who banishes his son from the kingdom of Asgard because he screwed up. He's a hot-headed, temperamental young man... probably a chip off of the old block but I decide he's not really ready to rule the future kingdom, so I banish him. I'm harsh and my wife complains and I say, 'That is why I'm king.' He's ruthless, take-it-or-leave-it. Women are much more forgiving; men are not so forgiving. I know in my life, my karma is, 'If you don't like it, tough, move on.' And I move on. I'm a little like Odin myself".[48] In May 2016, Mel Gibson stated he was approached about the role but "didn't do it".[49]
Additionally, Tadanobu Asano portrays Hogun, a member of the Warriors Three primarily identified by his grim demeanor and as the only member who is not an Æsir.[31] Ray Stevenson said of Asano's character, "He doesn't speak much but when he does, everybody shuts up. But also in the healing room where everyone licks their wounds, he's the guy who just goes about his business".[50] Josh Dallas portrays Fandral, an irrepressible swashbuckler and romantic member of the Warriors Three.[31] Stuart Townsend was initially cast after Zachary Levi was forced to vacate the role due to a scheduling conflict.[31][51] However, days before filming began, Townsend was replaced by Dallas citing "creative differences".[52] Dallas said he believed that Fandral "would like to think of himself a philanderer. He would like to think of himself, I was saying, as the R. Kelly of Asgard. He's a lover, not a fighter". Dallas mentioned that Errol Flynn was an inspiration for the character stating, "He was a big inspiration for the character and for me. I watched a lot of his movies and kind of got that into my bones. I tried to bring out that little bit of Flynn-ness in it. Flynn had a lot of that boyish charm that Fandral's got...."[50]
Maximiliano Hernández appears as S.H.I.E.L.D. agent Jasper Sitwell, Adriana Barraza plays diner owner Isabella Alvarez and Isaac Kappy plays a pet store clerk.[53] Joseph Gatt, Josh Coxx, and Douglas Tait portray Frost Giants.[54][55] Stan Lee and J. Michael Straczynski have cameo appearances as pick-up truck drivers,[56][57] Walter Simonson has a cameo appearance as one of the guests at a large Asgardian banquet,[58] and Samuel L. Jackson and Jeremy Renner have uncredited cameos as Nick Fury and Clint Barton / Hawkeye, respectively.[59][60]
