Rifleman: You are not a sniper or marksman, you're the main fighting force out there. You got the numbers and gear. Get as close as you can with the enemy and take the objectives together.
Autorifleman: You are somewhat the same as a rifleman... just with a fully auto weapon. Lay down supressive fire and clear out buildings.
Machine Gunner: Don't move up as fast and far and the rifleman and autorifleman's do. Stay back, and just keep shooting, even if you don't see anything. As long as you're shooting, the enemy won't come over which means the guys up front can get in easier. Call out for ammo by hitting the default key "V" and click "Request Ammo". If the team that is on is nice enough, they will get you some.
Sniper: Stay in the back and cover everyone. Keep the crosshairs on MG nest, windows, doors, etc. Go on the far sides of the map and be sneakt. once you get behind the Japs, you can pick them off like ducks. Aim mostly for their machine gunners first out of everyone. They are the huge threat as well as their SL's and TL.
Engineer: Destroy objectives, fences, barbed wire so you're team can move up easier and faster. Throw you're satchles in bunkers, and behind cap zones so you can blow a huge area up and possibly keeping back a few guys.
Flamethrower: You are very, very important. You may have to get close to the enemy but once you do, you got them. If attacking, clear out buildings and cap zones. If you are defending, stay crouched in the trench and when they get close, stand up and spray fire everywhere. Keep doing that so the enemies can't advance and you will most likely get a lot of kills.
Squad Leaders: (If you never played as a squad leader, do the training first!!!) Command you're squad around. You are incharge of everyone in you're squad. Stay alive too. You are the most important role out on the battlefield. As long as you are alive, you're squad members can spawn on you and get to the action much much faster. Give the team leader artillery marks so you can get artillery support for you're team.
Team Leader: (If you never have played as a team leader before, do training of it first!!!) You're job is very important too. Comand the team around. Stay near radio's and call in artillery behind the cap. Not on top or in front. That way you're team can move up as the artillery falls. If you have used recon and artillery and it's on the timer, don't be a worthless soldier, get away from the radio and fight.