Throughout the design process I toyed around with different ideas for the Corrupt Cop mechanics. There were several aspects to take into account...
Bluff and silent corruption
First, I had to decide how the Corrupt Cop could be able to mess with the work of the good cops without immediately exposing himself/herself, but still in a way that might hint that something is rotten, to not leave the good cops completely clueless of the nefarious actions.
I came up with the idea that the players would not reveal what Investigation Cards they were drawing; this way a Corrupt Cop could claim that he only found useless cards, when he/she in fact was discarding real evidence, thus delaying the crime investigation. At the same time, other cops might be unlucky and draw useless cards for real, but not being able to show these would leave them just as suspicious as the Corrupt Cop.
This part worked fine and made it all the way into the final prototype. However, it favored investigators working on the murder investigation itself, while the street smart patrol officers/sergeants would have a harder time getting away with messing with evidence, since this was not their strongest field, and thus it would seem suspicious if a patrol officer spent a lot of energy on the murder investigation (although some times this is necessary).
To fix this, and to add some variation, I added more ways for the Corrupt Cop to mess with the game. For instance, some character skills and Player Cards enable players to look at the top Event or Investigation Cards, and then choose whether or not to discard this (inspired mainly by the traitor element in Battlestar: Galactica). Once the top card is drawn from that pile, players could start accusing each other of trying to throw away good cards, even if it might have been an innocent player just trying to help by hopefully drawing and discarding a bad/useless card.
Corrupt Cops will also be able to influence the game in a lot of indirect ways, e.g. by not playing Player Cards when needed. The police might find themselves asking other players to play Player Cards to assist them when in trouble, and the Corrupt Cop can simply claim not to have the right cards. A few other examples of hampering police progress in a discreet manner would be counter-accusations (can be very fun), wasted use of resources (cards, officers, donuts, time) and bad priorities: “I think I will go here and arrest this single Street Criminal. Yeah, I know it will not ease the pressure on the city much, but it is the safest choice at the moment, and safety comes first, right guys?...”
Exposing the Corrupt Cop
To actually reveal a player of being corrupt, I decided that it should not be without consequences. I am forcing the accused player to reveal his/her loyalty, so if accusations could be easily done, players might exploit this and accuse left and right early in the game. This is where the Donuts come in: You must earn at least three Donuts (experience tokens in this game) to be able to accuse someone. You can try to convince other players to help you out in the accusation, by adding their Donuts to the accusation; but if you are wrong, you will have to discard the Donuts and skip the rest of your turn… and the Donuts lost will have to be replaced before you can accuse again, and they could have been spent instead to buy useful upgrades.
Revealed Corrupt Cop in action
Once the Corrupt Cop is revealed (involuntarily or not), he/she will go underground and try to hamper the police work in a more direct way. Initially I gave the Corrupt Cop the opportunity to either play an Event Card or distribute a few Street Criminals, but it quickly turned out to be too boring.
I then added a Criminal Underworld Board with different locations for the Corrupt Cop to travel between, to conduct the evil deeds listed for each location.
It worked ok, but it was too close to the Cylon ship in Battlestar: Galactica, and I often found people sticking to the same two locations - again not allowing for much play variation.
I finally decided on a Criminal Act deck, given to the Corrupt Cop, once revealed. It would grow in size whenever bad things happened. It provided more variety, and nobody would know what bad things the Corrupt Cop could throw at them…