Pay very close attention to your team. If you see someone doing odd things, such as not shooting at enemies when they appear, or moving rather far away from you when your walking, be on alert as this could be a sign of infection. However, do not waste a Blood Test Kit on him to see if you are correct. Just be on alert because they could mutate at any second. This also goes for you as well -- if your team sees you doing the same things, they will question your trust and may attack you.
Blood Test Kits
Save Blood Test Kits for showing your team that you are not infected to gain trust. Using them on a team member is useless because they get infected randomly, and usually break out within a minute of infection.
Defeating the second big Thing
Go into to his garage. Fix the electrical panel that operates the hoist. Step back towards the middle of the big door. There is an opening between two of the crates. Using your grenade launcher in first person targeting, send in a grenade. If the Thing turns its head toward you, then you have him targeted. Continue sending in grenades until he begins to burn. You can send in a few more if desired for extra fun. Run into the Thing's room and on the far wall from the big door is a switch. Throw the switch and watch as the Thing dies. You will probably take one hit from him.
Defeating the third Boss
Run around the Boss in a circle while shooting it with the machine gun. When it is low on health, use the flame thrower to finish it off. If done correctly, you can kill it without being hit. After you kill the Boss, the Thing in the glass in the room will break out, and the door will be opened with a few guards.
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.
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.
Hidden trailer
Wait about one minute at the main menu to view a longer, uncut television trailer for the game.
Keeping your team together
Listen to your team. They will tell you how they feel. If you hear someone say something similar to "We're gonna die", check the team menu and see how he is doing. If there is an X across his face while he is shaking uncontrollably, bring out the Tazer and shock him. Wait for him to calm down, move out of the area, and give him back his gun. He might be bitter, but after you kill a few more things he will be fine. Otherwise, he might start shooting you and your team, ending with himself in the head.
Medi-Packs
Always try to keep medics alive because medical packs can be few and far between. If you want your medic to heal a member of the team, tell your team to stay, then push your medic into the wounded team member. If he is hurt enough, the medic will heal him.
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.
Preserve Your Flares
You can preserve a flare and use it over and over as a portable light source, ignite one, but don't toss it away. You need to keep it on hand to do this glitch. When the flare is about to wink out (extinguish), highlight another object then go back to the flare.
Road tunnel
In the level where you first enter the road tunnel, go right to the truck. Blow up the barrel to kill all the little things. You can now either have an engineer fix the lights or not. Follow the left wall back to the door you came in and stop near it. There is a Walker locked up in some boxes. Throw a grenade to blow up the box and let the Walker out. Run back to the truck, or better yet, stand there and watch (if the lights are on). The guards not only get slaughtered, but really wear the Walker down. After he kills the guards, get him to walk around to you (not leaving the general area) then blow up the barrel. If you done correctly, you will kill the walker with ease. Otherwise, torch him.
Saving ammunition
Try not to give all your team members the same weapon because your ammo will go quickly. A good combination is you with a Shotgun or Machine gun, your second man with the one you are not using, then a Pistol. The rest depends on your ammunition supply.
Shutting doors
Always shut the door of the room you walk into, if you have the chance. For example, on the level where you have to kill all the walkers, a walker will enter from the front door if you do not shut it.
Skip first Boss battle
When you encounter the first Thing Boss, hold Up/Left to run diagonally and you should run under it before it can hit you. This will also aim you directly at the junction box. Immediately start fixing it. As soon as it is almost fixed, hold Right to run through the door. Since he will still keep fixing it as long as you do not release the button, with luck it will be fixed before the Boss attacks for the third time. At the intermission sequence you tend to take off in a random direction, so this may take a few attempts -- save immediately before trying this. If successfully completed, you will lose only about half a bar of health and no ammunition, making the next area much easier.
Unlimited flares
Highlight "Flare", enable it, but do not throw it down. When it is low, press Left or Right and highlight another object. Then, highlight "Flare" again. It will have restarted and you will not lose another one.