With this guide, you can be sure you won't be missing out on any content. Just what are these character-specific tasks, you ask? That's what the upcoming list is all about! I will provide a description of each target and the purpose they serve in the game.
Since you're about to permanently assassinate characters that have roles outside of Nazir's quests, you should make sure you complete everything that each target offers before accepting the contract. These targets are fixed, so if you're told to assassinate an NPC you've grown fond of. He has a limited number of quests, all of which involve unique characters. You don't have to worry about killing anyone important, so use these quests to brush up on your sneak skills. Fortunately, these are generated by Skyrim's radiant quest system the NPC you are sent to kill is always generated when the quest begins.
The Night Mother has an unlimited amount of contracts. Following the main Dark Brotherhood quests, there's an optional series of secondary assassination contracts given to you by Nazir and the Night Mother.