It is can be a good idea to reveal yourself as an Amnesiac if the settings "Cannot become Mafia/Triad" and "Cannot become killing role" are on. The Town will have no reason to lynch you, and evil roles will not want to waste a kill on an Amnesiac.
If you choose not to role-change, then you should play similarly like a Survivor - avoid drawing any suspicion upon you and promise the Town each day that they'll have your vote. Otherwise, if they lack leads, they will surely lynch you.
Another important thing to know is that you're basically a Survivor without any bulletproof vests. Avoid drawing any attention toward yourself or else a killer will definitely target you and you are guaranteed to die and lose the game.
The earlier you make a role-change, the more likely you have made a bad judgement on what faction you chose. However, the later you wait, the more likely you could be killed off before you could complete a role-change. When feeling threatened or being accused of being a Witch, you should become a Town role just to guarantee your innocence and draw attention elsewhere.
Being cleared from being a Witch is a huge advantage from a Survivor as a Survivor cannot prove his innocence in any way unless a Detective tracks a Survivor; the Amnesiac can just role-change into a Town role to prove his innocence.
Along with Coroner, the Amnesiac is one of the only two roles in the game who can target dead people.
If you have not selected any role and die as the Amnesiac, you will not win at the end of the game. If you die as a Town or Mafia role or Cultist (whether through remembrance or being converted), then you will still get points for your victory if your selected faction has won.
If you do not select any role, you must survive to the end of the game to win.
If Amnesiac chooses not to role claim, they can win with anyone, as long as they stay alive.
If the "New role revealed to town" option is enabled, an announcement will be made to everyone in the game when the Amnesiac chooses a role.
Amnesiac is one among the few roles that cannot die to Veteran.
Amnesiacs have role-block immunity (unless jailed) and mind-control immunity as long as they are an Amnesiac. As soon as they role-change, they lose these benefits.
If the Amnesiac takes the role from the body that has limited charges, he will inherit however many charges the role had left.
He will always have at least 1 charge (i.e. 1 shot if becoming a Vigilante).
He can use his new ability immediately, ignoring cooldowns the following night. (This doesn't always apply if the Amnesiac becomes a Cultist.)
If an Amnesiac becomes a Disguiser, even after a Disguiser erased his target, the Amnesiac is allowed to disguise again. It is actually the most recommended change if you decide to side with Mafia because if a Sheriff/Investigator find you out, you have the chance to escape a lynch while at the same time deceiving the Town.
See Disguiser for advanced strategies on this role.
If the Amnesiac gets doused by an Arsonist and does not know he was doused, then later an Arsonist dies without ever igniting, then if you role-change into that Arsonist, then you will not know if you were ever doused, despite the fact Arsonists always know when they get douse. If this is the case, then you should un-douse yourself (done by not take any action) the night following your role-change before doing anything else.