I was watching The Thing again and I always wondered when who was assimilated and such, especially since none of the prime suspects were, and how could the Thing could gotten to the key to the blood freezer. The FAQ on IMDB gave me some ideas.
In my opinion, Palmer was assimilated first considering that he looked like the shadow the dog approached and Palmer was probably high when the Thing attacked him. Norris probably wasn’t attacked first because he was offered to be the leader but turned them down, the Thing probably would’ve wanted to be in charge. When MacReady, Norris, and Palmer find the crash site, Palmer was probably infected, but didn’t wanna risk taking on both MacReady and Norris. Blair must’ve been assimilated from doing all the autopsies on the bodies after Palmer, considering that he smashed up the chopper and used the pieces to build the ship later. Also, in the scene where Bennings was attacked and Windows caught him, he dropped Gary’s keys, giving Blair, since he’s the only one absent, the perfect opportunity to get to the fridge and put the keys back where Windows dropped them. Blair went on his spree so that they’d isolate them from the group to build his ship and divert their attention to Clark. Norris was probably infected third, probably by Palmer when someone blew the light out in the lab, they said it would’ve been the perfect time for someone to infected when they suspected MacReady. One of the two tried to assimilate Fuchs, but he burned himself before they could. Then one or both burned MacReady’s clothes to frame him. Blair was probably still building his ship. Norris’ copy probably imitated him so perfectly that it imitated his heart condition, Norris was even acting weird before everyone turn on MacReady. He probably bit off the Doc’s hands because either the defibrillator was hurting him, or the copy of Norris was messed up he had to abandon the body and move onto to another. After Palmer was discovered, Blair probably realized he wouldn’t finish the ship in time and destroyed the generator for his backup plan.
Well, that’s it, that’s my theory. Whether Childs was infected, I cannot answer that. Blair could’ve gotten to him. We’ll leave that up to you guys.