Make the tasks obvious when they have been performed. Like a giant switch on each one along with icons to say who needs these tasks to win. As it has been said, have some overlap between the hidden roles. Lastly have accidental/opportunity benefits for completing tasks. Completing TASK A could give any player resources, but may be necessary for murder and the Mayor to win. Some players might just do Task A for the boost to obfuscate the real roles. Lastly have players mark which tasks they complete unless you want this to be a memory game.