It can be really easy to write a simple point A to B quest and lean heavily on the uniqueness of the monster to make a session interesting. However, by throwing a twist into the mix you can force your players to make more thought out decisions.
It also isn’t difficult to implement. First, identify the primary quest.
EX: ‘The heroes are hired to slay the banshee on the hill.’
Then, throw an interesting twist in that runs adjacent to the primary quest. It could be anything; the reason the monster is there, an odd double-cross, a hostage situation, etc.
EX: ‘The banshee is lashing out, calling every male adventurer ‘Arno’, Arno murdered the woman that became the banshee and still walks the city streets free.’
The whole point of the twist is to make the problem more dynamic and feel more real. The heroes could talk to the banshee, capture and bring Arno to justice, and allow the banshee to pass on in peace. In contrast the banshee could be expelled from the mortal coil and Arno is left to go free to cause more mayhem. No matter which course of events happen, by simply adding a twist to the standard monster on the hill story, you’ve made it far more memorable for your players.