Here's an article about the battle roles we will take in Helms Deep.
Within each battle players can choose from three featured roles:
Vanguard: Hack and slash, mainly a traditional combat role. You’re in the thick of the fighting and engaging with enemies as they come over the wall.
Officer: Strategy and command, you’re issuing battle orders, healing and dealing with your troops.
Engineer: Interacting with the siege weapons in all the various ways possible, from loading and prepping them to fire, to repairing them when they break down.
These roles aren’t like character classes, but rather occupations for the duration of the fight. Each occupation determines how you will participate, but you can try something different the next time around. Choosing to be an officer in your first play-through doesn’t mean you have to be an officer every time after that, or ever again if you don’t care for it.
As each battle progresses, there will be advancement within the occupations, and you can choose traits from any of the three skill trees. So, Officers can potentially have Engineer skills, Vanguard can issue orders, etc.
Aaaannnnd freak out in 3....2....1...go!