This is an offshoot of the Conquered! campaign seed page. it provides the GM with some ideas for what to consider in developing the campaign theme.
One of the things you need to work out is where the resisters are most likely to be able to meet without being observed by the conquerors or their collaborators. In the Second World War the French Resistance often used churches as safe contact points – they would rarely do anything to break the neutrality of the Church (like storing weapons in the crypt), but they knew that the German Army did not keep churches under surveillance, and anyone could drop into a church at any time for private prayer or to discuss personal problems with the priest, so it was not a suspicious place to be seen entering. Strangers arriving in town (like downed airmen) could often contact the Resistance by talking to the priest, who knew what was going on in town and usually wouldn’t betray them to the Germans.
Helping resistance forces is always risky. Players should realise that good charisma is vital in at least one of the group, so that they can persuade nervous neutrals to become at least covert allies, or talk their way through vehicle checkpoints without a search. Putting a lot of such people in their way in the early days will help them see the urgency of putting their earned points into Diplomacy, Fast-Talk, and similar.
One common response by evil occupying forces to resistance is the taking and executing of hostages. If it becomes known that ten or twenty randomly-selected people from the nearest town or village will be executed for each ‘terrorist incident’, those who are resisting suddenly find themselves a lot less popular. They will often be betrayed by the relatives of those being held hostage (the terms of hostage taking being that “if we don’t have information about who committed these crimes in 24 hours, we will execute the hostages”). Players will have to decide what to do in response to this tactic – a very grey area ethically. Especially if they know the hostages.
If the PCs are captured, it’s not the end of the campaign. They may be sent to the mines/galleys etc, from which of course they have a chance of escape. You will need a good reason not to have them immediately executed, of course: perhaps the occupiers have a sudden need for a certain material which makes slave labourers more valuable than minor resisters being executed to set an example. If one or more PCs are not persuasive enough to talk the Commander of the enemy into commuting their sentences, perhaps a local priest can do so.