In ‘The Day of The Doctor’ the Zygons and the humans they are impersonating can’t remember who is the original and who is the copy. This isn’t the first time that ‘Doctor Who’ has explored the concept.
Most notably ‘The Rebel Flesh/The Almost People’ revealed that the Doctor and his ganger had swapped places, to test how others reacted. That same story also revealed that Amy was actually a ganger (although one remotely controlled by the real Amy).
Rory struggled with his identity in ‘The Pandorica Opens’ when he was resurrected as an auton. Despite his best efforts he was unable to prevent himself from shooting Amy. Not dissimilar to how some of the Cylon sleeper agents in the new ‘Battlestar Galatica’ were unable to fight their own programming.
These uses of identity work for the viewer because we are an observer. We aren’t in those character’s shoes and we can be tricked by appearances. Would we feel the same if we were inside their head?
In a roleplaying game a player assumes the role of a character. Their expectation is that they know who that character is. That they have all the knowledge that their character does in order to make informed decisions.
Revealing that they aren’t who they thought they were can have even more of an impact but must be treated carefully. The last thing you want is for the player to feel like they’ve been tricked.
This works best where their double doesn’t know they aren’t real. In this scenario the player shouldn’t be playing their character any differently than normal. When the truth is reveal the player and their character should be shocked.
If the double is a sleeper agent they might act completely normal until a key moment, in which case the games master takes over as it is unfair to expect a player to do something they disagree with. The player could resume control of their original character or they might take back control of the shape shifter once the personality they are copying returns.
The double might be just very good at impersonating others, while still retaining their own mind. In which case having a player take that role demonstrates just how good the double is. This is where players can feel they’ve been betrayed and that they’ve put others in danger because of information they didn’t have.
It is important to think about what your purpose was in having a player take the role of a double.
In ‘The Day of The Doctor’ it is about putting a character in the position of another, because they don’t know who they are. They must believe that their double (whether it is the original or the copy) feels and thinks the same way.
This can give players a greater appreciation of those that they face. They might be alien but they are people (‘different casing, same software’). Going forward this might alter how they deal with others (looking for peaceful solutions rather than resorting to violence).
In ‘The Rebel Flesh/The Almost People’ it is about how people react to the double. What does it feel like to be treated differently because you’re an alien, synthetic or copy even if you are? What does it feel like to treat someone else like that, even if they’re not?
This can challenge preconceptions. It can make a player character (and the player) realise that they do judge people on what they are, rather than who. It could also make them realise the hostility that their enemies usually feel.
‘The Pandorica Opens’ uses the change in identity for the purposes of horror. Rory is trapped in a nightmarish situation where he knows that he will kill the woman he loves and there is nothing he can do about it.
This can be a tense situation to put the player characters. If they know they will hurt others or do something that is repellent what will they do to stop themselves? Do they believe they have the willpower to overcome their programming or will they try to destroy themselves? Will their true nature let them?
Having a double assume the personality of the player character can be a tale of redemption. Could their influence change the double for the better, dissuading them from their evil plan or letting them overcome their instincts?
You should think at what point does the player assume control of the double. If your adventure starts in medias res, with the switch already taking place. By closing over the point at which the double began thinking they were the original you keep it hidden for the players.
The change might happen at some point during the adventure. This is more likely to catch the player by surprise. You just need to find a point for the switch to happen, such as between scenes.
If the player characters are already aware that doubles are involved in an adventure then the switch could happen when there is some confusion. Whether it be a temporary memory wipe or a blackout, you open up the possibility that characters have switched places and even the players can’t be sure who they are. This can increase the level of paranoia if the doubles have been shown to be hostile in intent.
Players will want to resume control of their original player character going forward. This is usually a good idea, unless you want to have something really shocking like killing the original and having the double continue in their place (which works best if there is very little difference between the two).
You should think about what happens to the double, which will primarily be determined by the players actions. Is their double destroyed, left behind to live in peace or could it join them in their travels?
You could leave their fate more open ended, so that the player characters don’t know what happened to them. They could return in the future (maybe the PCs hear about things they’ve done but don’t remember and have to wonder if their double or something they’ll do in their future) or act a replacement should a character die (always good to have a spare copy around).
At the end of the adventure the player might still not know if they are now playing the original player character or their double. This question of identity could be an ongoing character arc.
There are plenty of alien races that this would work with. Zygons are the obvious inspiration but also autons, faceless ones, Sontarran clones, Rutans, gangers. You can also create a new species or automaton capable of copying others.
Surprising players by having them take the role of doubles can be powerful but must be used sparingly. More than once and players will constantly be questioning just who it is they are playing.