I think characters are given random roles in cutscenes, but i think singleplayer, you're character is in the background doing various tasks, or says very little.
In co-op and nultiplayer, characters get rabdom spots with pre set actions and lines, like in KZ3 operations mode.
If you are playing in "your" campaign (with "your" matchmaking), your character will have the main role. You're sort of playing his story in the whole war, after all.
If you are playing online, spots will be given randomly, but kept throughout a mission.