You do not visit your second target, so you won't be affected by a Veteran on alert, a Crusader, a Bodyguard, a Trapper's trap, an Ambusher, and so on. However, you do see anything that happens to your ghoul, so if they encounter one, you will know.
If your target was an Amnesiac, the message, "An Amnesiac has remembered that they were like the Amnesiac." will be displayed to the Town. This does not seem to depend on the role of the second target.
You cannot control a dead player if they have been stoned or if their role was cleaned.
Use killing roles or anything you think can mess up the Town.
If you think that someone in the Coven or an ally is going to be attacked, reanimate a TownProtective role.
Keep other powerful roles with the abilities to kill, protecting or healing in the late game. This also provide sustain to your faction in late game especially when you are the only member in your faction.
You can choose to select yourself to summon ghouls and continue to use dead corpses if you desire.
Crusaders can be your toys to kill enemies as well. Using their ability to visit important members like the Mayor and Psychic can help you kill members trying to protect or watch over them.
Controlling the Vigilante can be used for deception. Accidentally killing an evil role makes you able to claim Vigilante. After all, the message will say that the Vigilante killed them. Other killing roles, like Crusaders, can be also used for deception into tricking the Town to make them look like you did the job.
You can control Pestilence if he was hanged after transforming, but be warned that this WILL kill you as well! Only use this as a last resort if you believe it will potentially push the game in the Coven's favor.
If you see that the Jailor has imprisoned a Coven member, sending the Doctor to heal the jailed target will not prevent an execution.