To jump in on the issue of having separate actors as Blake and Thor... The basis could be that Thor's consciousness is inserted into Blake's body as Blake dies (perhaps a car accident, to explain the gimp and cane). Show a scene of Thor being banished, and then a first person view for the movie-goer, soaring over the sea and land closring in on Earth. We slowly decrease in altitude, so that we can start to hear the ambient noise of the city below... the sounds of sirens begin to become noticeable... We get very close to ground-level and see firemen trying to get someone out a twisted car-wreck, saying that there are no vital signs...
Then suddenly, the guy sits up sharpy with a gasp. We cut to the hospital, and the guy (we dicover is Dr. Blake) is having trouble remembering what's happenning. The movie moves forward with the Blake character re-adjusting after the accident and getting back into the swing of being a doctor. Everyday that he faces a medical situation is like a new revelation to him (of challenge, and drama, instensity, and a lesson in the human condition)... and interspersed are scenes where he wakes up in cold sweats, from very vivid dreams of battle scene scenarios and Asgardian monsters. He eventually discovers that he is Thor within the body of a fallen Dr. Blake, and he had been sharing some of Blake's memories (in "returning" to life as a doctor)... similar to how Robo-Cop shared some of the memories from the Peter Weller cop character, whose brain was used to build the Robo-Cop cyborg.
This way, we can assocate the different actors (scenes as Blake and scenes as Asgardian Thor) to the same character... He could even return to being Blake at the end of the film... until in the Avengers film, where SHIELD (who has been monitoring Blake for a while, after having secretly observed the final battle-sequence from the Thor movie) approaches Blake and say that they know who he really is... and want to talk to him about the Avengers.