Okay, I still don't get how the aliens take over the captain and the female LT....? They didn't die apparently....and they didn't fall asleep, either...so how did they get possessed? They are still 'human' when they are almost back to the ship (she almost falls into a lava pit and screams).
shareWell the easy answer is you don't know that for sure.
They two run off with the deflector and a few minutes later they run out of the mist and back to the ship. But if you remember the two humans give them cover fire and are killed from behind, proving the aliens were already surrounding them but chose either:
A) Not to shoot the escaping humans when they easily could have or
B) Did kill the humans and took them over off camera.
You don't see the characters at all moments in the film so you have to assume they were turned at some point off camera. It's a simple explanation but it's the only one that makes sense. As for the woman screaming when she almost fell, she just had a normal reaction for any creature. If she was an alien at that point she still would have died in the lava or at the very least destroyed her human body which is the only way to escape the planet.