Unlock achievements or play Matchmaking a lot.
Play through the campaign levels on normal and you will be able to unlock them. The one firefight character you will not be able to unlock is Sgt. Johnson. In order to unlock him you will need to pre-order Halo 3 ODST.
No, they are easier to unlock.
no
You have to get certain achievements that unlock certain armor permutations.
You Can't
head-unlock overkill achievement shoulders-unloked by default with the new update body-unlock triplekill achievement
You Can't
To get the Katana as an armor permutation in Halo 3, you need to get 1000gp in the game. The new achievements released for Halo don't affect it. The easyst way to get it is to get all the "old" achivements.
You Can't
For now, no. Although, in the near future a new kind of armor might be introduced, much like Recon armor was in Halo 3.
Every week, new characters are unlocked. You don't have to unlock them. yes you do have to unlock them ! every week theres a new one click on the character to see what you have to do to unlock him . for exampel ultimate humugisouar you unlock him by playing 3 matches but you have to be loged in
By going through the Story Mode over and over again. Each time you finish it, you'll unlock a new character.