The last time we saw Thor (played by Chris Hemsworth) was at the end of Thor: The Dark World, where he had chosen to forgo returning to Asgard and instead stay on Earth. He's now about to pick up Mjolnir a fourth time, with a threat that Hemsworth describes as a story that raises the stakes for the Avengers without going too big to lose sight of its grounded conflicts, all courtesy of Joss Whedon. "I just loved how it upped it in a way that wasn't just bigger and flashier. Everything had been amplified, but in an intelligent way," said Hemsworth.
In a recent interview Hemsworth said that this movie we will also see a more laid back, relaxed Thor, who has been growing more accustomed to life on Earth. "This time there's more humor in Thor," says Hemsworth, "because he's been on Earth, [and he's] a little more accessible now. He's off Asgard now, so he doesn't have to be as regal and kingly as he is in that world, which is nice. I enjoy that more. Here you can have a gag with the guys and he can throw away lines and be in a party scene with them in civilian clothes."
Apparently during that party scene he walks in with a nice coat and jeans and everyone is jokingly asking how Thor went shopping, or if it was his Girlfriend Jane (Natalie Portman) who dressed him up.
Now to the more interesting tidbits that Hemsworth revealed:
I kept saying to Joss, 'Okay, what do I bring to the table, besides Thor being one of his foot soldiers and the muscle and the fight scenes? What is his knowledge he can bring to it? What information can I bring?' So [Thor] calls upon some of his guardian knowledge and is able to go into another realm to pull out some information that's hugely useful [and] certainly benefits where they are at that point.
What other realm does he get information from for fighting Ultron? Does this mean that Ultron isn't flying solo and might be somewhat in connection to other players on the giant board that is the MCU? This might tie in with whatever vision he sees courtesy of the Scarlet Witch. We already guessed that Scarlet Witch uses her powers to plummet the Avengers' into nightmare like sequences that make them face their worst fears, and now we're told that it is something that will help open Thor's eyes to the larger threat.
I think [the Avengers] will begin to have their fears held up in front of them. For Thor, I think it's a corruption of power. Trying to have the understanding that we're in this endless battle here and wondering when this is going to end, and how does it end. That's where he really starts to kind of move through the story. Once that dream occurs, he can see what's coming. It's a ticking clock.
Hemsworth also stated that it isn't just the Hulk that Tony is going to be going up against, promising a fight between Iron Man and Thor that he describes as a "lengthy fight scene of destruction". It sounds like Iron Man is going to be spending a lot of screen time fighting his own teammates, unless he is taking on the Hulk and Thor simultaneously while in his Hulkbuster armor.