They will wait until an imperial caravan,patrol,etc and attack them yelling "Death to the empire" and "Long live skyrim". They will be something like bandits but will not attack the player (unless dressed like a legionaries) or rebels.
Some will look like regular people others like regular soldiers.
they will have different names depending on their groups , such as.
the white mountain brigade guerrilla fighter or the fearsome heart guerrilla fighter .
The only speak able members would be leaders who offer you gold for the amount of legion insignias you bring him. Get enough and you get unique Armour or titles. and bonus relations with factions.
The regular fighters would say "shhhh I'm waiting for legions" or " have you seen any legionaries around?" and the odd " talk to the captain"
I think it will add immersion into the civil war.