Unlockable: Free Weapons & Difficulty
Too cheap to buy the weapons? You can unlock access to all the secondary and special weapons by beating the campaign on professional difficulty, or just the primary weapons (assault rifles, shotguns, etc.) by beating the game on any other lesser difficulty.

To unlock professional difficulty, do as the game tells you -- beat the game's campaign on either recruit or contractor difficulty.

Unlocked weapons are only available at their base (no upgrade) version.

Hint: Briefcase Locations
Below are the locations of all fourteen intel briefcases in the game. Please keep in mind that they can only be collected one time, so if you've collected a briefcase, it won't appear again on a subsequent playthrough. On the flipside, however, if you didn't collect them on your first playthrough, they'll be there on subsequent playthroughs until you do.

Afghanistan

1.) Look behind the initial M-11 missile you encounter. There's a small path going uphill near it. That's where this intel briefcase is.
2.) Kill the first heavy armor guard you encounter (there will be a cutscene), and search the platform area nearby. The briefcase is on top of a short set of stairs at the back end of the platform.
3.) In the area adjacent to Brian Hick's cell, you will find this briefcase.
Iraq

1.) Right before climbing up the ladder in order to take out Ali Youseff's Helicopter, grab the briefcase at the bottom of the ladder.
2.) After calling in the air-strike on a nearby bunker, seek out a large tent near the dividing wall. The briefcase should be against the wall near the tent.
3.) Right before you meet Ali Youseff, there are some metal platforms with armed guards on it. Kill all of the enemies, and then climb up the steps to the platform's second level. Climb another ladder to a third level, where the briefcase is located.
Aircraft Carrier

1.) After landing on the ship's main deck, run to the right of the control tower. The briefcase is nearby.
2.) When you enter the hangar to use one of the parked planes to blow through a door, seek out some missiles. Look behind them, as they obscure the location of this briefcase.
3.) After setting the missiles to blow up the ship in the final hangar, look around to find a sort of makeshift basketball court. Behind the farthest basket is this briefcase.
China

1.) After arriving at the dam, head towards the first control tower. Across the dam is a similar control tower, where the briefcase is located.
2.) After you arrive at the village and co-op snipe some enemies, choose to take the rightmost path. When you come across the first house you can enter, do so, climbing to the second floor to find this briefcase.
3.) When you arrive at the factory, don't enter it immediately. Instead, search around and behind the factory to find this briefcase.
Miami

1.) Once you've opened the gates to the luggage area, you'll need to head to the control room. There's a smaller office near the control room where this briefcase is located.
2.) When you're on the third floor of the SSC Tower and must operate a lift, ignore the lift and run to the far end of the third floor, where the final of the fourteen briefcases is located.

Hint: Panda Express (Money Farming)
Not really interested in dying over and over again because of the lack of cash in the early stages and don't mind the monotony of MMMORPG farming? Try a new campaign in China. The first objective pays $30,000 and can be cycled quickly (about 3 minutes with co-op snipe). The checkpoint is back down the hill next to the hovercraft. Once you get the checkpoint (saves your money), quit and restart the stage. Tedious but effective. You only wish you could make $10,000 per minute.

To skip the pre-rendered opening stage movie, press SQUARE (or TRIANGLE).

Just as an amendment to this technique (which works really well). If you set your partner to Aggressive Advance mode (by hitting up twice), he will run forward and clear the way for you all the way up to the location where you have to snipe at the bridge. Then, as you're sniping the bridge, he'll run back down towards the checkpoint that must be hit before you restart the game to do it all over again. It's not necessarily a faster way to do this, but it does require a lot less walking.

Hint: Refill Your Ammo
If you're running low on ammunition (or have run out completely) on any of your currently-equipped weapons, wait until Mid-Mission Shopping, which is an option multiple times on almost every stage in the game. Change your weapons when this option comes up, and the new gun or guns selected will be fully stocked with ammunition.

Hint: Heavy-Armored Guards
Heavy-Armored Guards are a pain to kill in this game, since you have to shoot them from behind to damage them. However, the powerful fifty caliber sniper rifle can knock back and knock down these guards. When they’re on the ground, they’re prone to any attack.