Thor's look in Avengers: Infinity War was originally based around a more "serious" version of the character, according to an artist who worked on the film's concept art. This would have been more consistent with the character from Thor and Thor: The Dark World, before Thor saw a big tonal shift in last year's Thor: Ragnarok.
The first two Thor films were not among the most popular films in the MCU (this was especially true of Thor: The Dark World, which remains the worst-ranked Marvel film on Rotten Tomatoes) and the character himself was often described as overly broody and lacking the same charisma as other characters in the franchise. This changed last year with Thor: Ragnarok, which saw the series get something of a soft reboot as a comedic action film in the capable hands of director Taika Waititi, a well-known comedic filmmaker. Not only did the film prove popular among audiences and critics, some argued that it saved the Thor franchise. As it so happens, though, Thor: Ragnarok's concept artists weren't originally aware of this abrupt tonal shift.
This change in tone turned out to be a success, and it was the right move, not only for the character, but actor Chris Hemsworth as well, whose brush with comedy in movies like Ghostbusters, Vacation, and even The Cabin in the Woods translated well to Waititi's new direction. This change extended into Avengers: Infinity War, wherein Thor's comedic interactions with the Guardians of the Galaxy proved to be well-received by audiences. However, the film's concept artists were under the impression that Thor would remain the brooding, serious character he was in the first film. In fact, according to Marvel's Avengers: Infinity War - The Art of the Movie, a concept artist on Avengers: Infinity War was in charge of Thor's design well before Thor: Ragnarok was even released, so he was unaware that the tone would change so significantly.
While this more comedic take on Thor proved popular among critics and casual audiences, Thor's personality - as well as the overall tone - in Thor: Ragnarok did prove divisive in the comic purist community. The shift in tone lent to some objection to how heavily the movie relies on humor, despite the fact that the film's predecessors both subjected Thor to comedic "fish out of water" moments. Thor: Ragnarok was just more blatant in its use of comedy.
It's entirely possible that after the tragic events of Avengers: Infinity War, in which half the universe is wiped out by Thanos, fans may see a return to a more serious depiction of the character. This could be especially true since it was Thor who failed to deliver the killing blow to Thanos with his new weapon, Stormbreaker, right before the Mad Titan's fateful finger snap. That said, given the positive response aimed at Thor from the general audience, it's unlikely that he'll stray too far away from this newfound lighter side.
Source: Marvel's Avengers: Infinity War - The Art of the Movie
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