I spread my abilities through all classes, tend to swap around quite often depending on what I think the team needs...
I go Soldier when we need to clear a defended area, heavy grenade usage to make it easier for us to get the kills
I play the Engie more offensively using most of my supply boosting my allies
I pick Medic when we are on defensive ops, so my team has more staying power
I grab the Operative when we're on the larger maps, I find it easier to make use of the disguise when the enemy players are more spread.. I try and down guys and comms hack as often as possible so my team knows whats coming
I change weapons quite often as well, some work better with specific classes, the engie/soldier specifically.. and also on certain maps favour different guns
Kinda wish you could change body type mid game, but I can see why it's not allowed.. I tend to stick to mid so I can cover most areas
Playing Engie with the gatling gun + large body type is a blast