As said by another poster, it depends on the organiser/game.
In ours for example, its optional for the players. Some like to put on Russian accents and get into character and some dont. Fair enough - its their game.
We do have staff playing characters. Last game, a peasant woman threw a cabbage at the guy playing the Prime Minister and a riot kicked off.
Sometimes as a player, you can stand back and be a spectator. Other times, its more difficult. On the op before, the government patrol at a crossroads were shocked to see a farmer on a tractor roll up with an IED round his neck pleading for help. Not so easy to ignore.
[name=Aunty Pasty" post="215236" timestamp="1442676954]One thing I need to ask regarding Milsims. Does it involve role-play? As in the LARP type of role-play where the players act or say certain things to maintain suspension of belief.