In the level where you must find Faraday, do not hit the switches until you have met the medic downstairs. Bring him up to the top of the ramps and tell him to stay. Hit the switches and go to find Faraday. Leave, and on your way out, the medic will not have turned. You will get two medics for the remainder of the level.
Hint: Get a doctor to follow you into a fight with the Thing:
Use the following trick to get a doctor in the same room with you during your battle with the second giant Thing. Walk to the large garage door. Get the doctor close to the door as well. Then, tell him to not follow you anymore. Push him as close to the door as possible; and close enough to you that when you flip the switch to open the door he will not move. Before you flip the switch, tell him to follow you again. Flip the switch and when you are shown in the locked room with the second giant Thing, he should be beside you. Use him to heal you during the fight with the Thing.
Wait about one minute at the main menu to view a longer, uncut television trailer for the game.