According to Nikki Finke (who broke the story), this is how it went down:
"The way Chris Hemsworth got the part of Thor is one of those great Hollywood backstories that happens only once in a blue moon. Ward had found Chris during one of the manager's many scouting trips to Australia. Ward brought him to Los Angeles and really put him out there to casting directors and production executives.As for the major agencies, I hear CAA passed on the meeting, Endeavor took it but passed on repping him, and ICM was interested but dragged their feet. But Eileen Feldman got his appeal immediately.
Chris had read for the part of Thor but wasn't given a test because a casting director had nixed him early on. I'm told Chris' younger brother Liam (who's also a ROAR client) then tested for the role of Thor, but Marvel's Kevin Feige passed. Then, after a conversation with Ward ('You've got to reconsider Chris, he's your guy'), Feige decided to let Chris read again. And once Marvel put him on tape, it was 'Oh my god.' Branagh came to town last week and saw the Chris test and made the final casting decision today."