Basic Mafia Roles:
Cop: The cop is for all intents and purposes a regular townie, but at night has an "investigation" ability. The investigation ability is targeted at any player, and that player's alignment is revealed to the cop.
The cop is a tricky role in that although you have information, there is no absolute reason for the other townies to trust him/her. Additionally, the cop is a fairly strong role who is often targeted by the mafia as a priority. So, a cop is trapped in a game since his knowledge is always looked at with skepticism by everyone other than the mafia. Furthermore, there are many variants of the cop role that the cop is never informed about. These variants can completely skew your results. Paranoid cops always scan other players as "guilty", naive as "innocent", insane as the opposite of the true alignment, and random cops as completely random returns (usually a 1 in 3 chance).
The conclusion to these many aspects is that the cop is most formidable when he has both identified the alignments of living players and he or she is also dead, so that the town may know what validity of scans the cop received.
Additionally, it is important to remember that you cannot claim on the final townie day. If you survive, without being outed on a "lylo" (lynch correctly or lose) day, and you decide to claim cop and point at your targets, there is nothing stopping a mafia from counter-claiming (claiming to also be the cop), and furthermore, if you or another are a cop variant, it can end the game in a loss fairly quickly.
Avoid these tells as a cop:
Voting/F/Hos'ing only one player per day.
Voting/F/Hos'ing with surety, or saying "100% sure"
Focusing on one player
Asking for protection
Doc-tells
Doctor: The doctor is another very common role with an ability to nullify a night-kill by "protecting" the correct person. At night, pick a player who you think might die, if that person is also targeted by the mafia to be killed, you will keep that player alive. The doctor, for one, can never, I mean NEVER protect themselves. A doctor who can protect themselves is a confirmed townie by all measures and can really disrupt the balance of a small game.
The doctor is trickier to play than the cop because the doctor has no protection. If a cop is discovered, or "outed", the doctor can always protect that person to ensure the cop survives, but there is no such safety net for the doc. Additionally, the doc is usually the first introduction to yomi play for both him/herself and the mafia. Yomi will be discussed in another section, but it basically refers to a cyclical "outsmarting" by opponents. For example, as the doc, you notice the cop has claimed. You know that the cop is a priority target for the mafia, and the mafia knows he is a priority target for the doc. However, the mafia knows the cop will be protected, so they might decide to "outsmart" the doc, and kill someone else. The doc, realizing this, can decide to protect someone else. However, if the mafia decides to kill the cop, and the doc thinks he might be getting outsmarted, it looks incredibly bad for the doc. Additionally, you must keep in mind that you are a doc, NOT a cop. If you protected someone, and there was no kill, it does not automatically ensure that your protected person is a townie.
Also, as opposed to the cop, the doc is completely useless once dead, and it usually means the cop's imminent death as well. Therefore, a doc wants to stay out of the spotlight while also appearing as town as possible without being overly productive so that they are targeted by the mafia for a night kill. The doc, as you can see is very much a role of balance, both in decision making and appearance.
Avoid these tells as a doc:
Lurking, being defensive, trying to blend in. It's not surprising that most docs end up on many players' lists of "possible mafia".
Mentioning the possibility of a doc/including the doc into various calculations/carrying a scalpel.
Cop-tells