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.

Persuade a person:
When someone's trust is in the orange and they will not do anything for you, enter first person mode and aim the gun at their head for three seconds. They will now do things for you without good trust.

Get Whitley to follow you:
The following trick requires Stun Grenades (found in the back of the room with Boss 3), Blood Test Kit (found in the same room), and the Grenade Launcher (in the save room down the hall from Boss 3). Starting after third Boss fight (hanging tentacle monster), backtrack your way out of the weapons lab, back up the air vent, and to the "mine" elevator. As soon as you reach the surface, strafe left, and equip both the Grenade Launcher and Stun Grenades. If you step forward at all, it triggers an intermission sequence you have to die and start the level over. Enter first person mode and aim up the corridor, parallel with the bottom of the last metal wall panel. Launch a grenade and quickly walk forward. The intermission sequence will trigger, but Whitley will stand still instead of running off. Now, quickly run ahead of him (preferably block his path to slow him down) and use the Blood Test Kit. He now trusts you enough to follow orders. Order him to follow you (otherwise he walks around the corner triggering guards to attack) and give him a weapon. You have now backup. He only lasts a brief amount of time, and will disappear roughly halfway up the snow corridor, which appears scripted. However, every bit of backup firepower helps in that spot.