Dungeons and Dragons becomes a communal story, and as any story, it soon has a life of its own. The player characters WILL do something entirely unexpected, go through plotlines out of order, and even have reactions the players themselves are surprised by. To combat the unexpected, try to have some back-up “plan B” plotlines.
For example, the party kills a key member of the plot early, well… maybe now they find key pieces of information on the leader’s plot or keys to hidden locations. Maybe that person is revived by magic later on & now has a personal vendetta. Also, feel free to throw barriers in the party’s way. Say they went sideways instead of forwards and then BAM – they serendipitously locked a magical door. They can either examine it giving you some time to think or they may mark it for later and continue forwards on the planned route.
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