When you are in 'Campaign' mode and you want to switch character go to the 'Campaign' menu and select 'Start New Campaign' followed by the character you want to play as. Then return to the 'Campaign' menu when you now choose 'Continue Campaign' you will start with the character you selected.

To go with this downloadable Challenge pack there are four more achivements, they are as follows:

Barrel SSC Challenger (25)
Destroy 500 barrels in ranked SSC Challenge matches

SSC Challenge Marksman (25)
Achieve 75% accuracy at the conclusion of a ranked three round SSC Challenge match

SSC Challenge Win (25)
Win a ranked SSC Challenge match

True SSC Challenger (25)
Win every stage of a ranked three round SSC Challenge match

These achivements are only available with this SSC Challenge Pack.

If you are able to complete the game in professional difficulty you will unlock all the secondary and special weapons. Pro difficulty is unlocked after you have completed the game in either recruit or contractor difficulty

After you have completed the game in either recruit or contractor difficulty all primary weapons will become unlocked when you play through the game again. It also unlocks the professional difficulty mode.

To unlock this mode you just have to complete the game once on either recruit or contractor difficulty.

There is a little trick with which you can accumulate quite a bit of cash, you just have to be prepared to do the same thing over and over again... Ok, here is the deal.

In China, complete the taasks at the beginning of the level, blow up the bridge and get to the hovercraft. Here the game will auto save your money. Now quit out of the game, reload and go back and do it again... And again... And again... Well as many times as you want.