Don't be unreasonable.
The DM won't whip out the most powerful monster in the manual to fight some level 1 Bards, and the players won't hijack the plot.
"A quest to find lost treasure? Nah, I'll just be here with these busty wenches. Then I'll burn the tavern down."
Although both sides tend to alter the deal.
DM's might force players to go in the direction that makes the plot continue, and the players might have a bit of fun saying "no" to the super awesome quest in favor of running around catching chickens.
Speaking of chickens....
|The most invaluable team member.|
He was an elf, turned into a chicken by a cursed necklace given to him by his evil father, who wanted to profit off of the destruction of the forest.
And so, the band of heroes abandoned their quest to restore the magic elemental crystals in favor of defeating this guy's dad.
This was not what the DM had planned.
Of course, the player... let's call him "Kevin," was a master at this. Apparently, he once created a character who was a donkey. And he joined the game when the party had to climb up a sheer cliff.
In the end, they pulled him up with a pulley system that they spent at least one real life hour figuring out how to craft.