Just as with life, sometimes things in fiction can take an unexpected turn. If you follow your intuition and, more importantly, follow your characters, you might find yourself somewhere you didn't expect.
The most important thing for me when I'm writing, is to follow my instincts. I never stray from this rule, but if I do, I hit a wall. Characters need room to grow and room to move. Sometimes, the place they go may not be what you expected when you start to write a chapter, but this can also take you to some very interesting places. And the way I look at it, if you're surprised where things go, chances are good the reader will be too.
What does this mean exactly? And more so, how do you 'follow your instincts' when writing?
To begin with, I've always said that when you sit down to write a chapter, it's important that you have a quiet (unless you listen to music) environment. This helps you keep focused and in tune with your inner voice. Writing shouldn't be something you overthink. It should flow and be natural. You shouldn't think about what you have to do later, what's going on outside your window, or what's on Facebook. You need to focus.
I never really outline chapters. I have very general notes and an idea of where things will end up. If I'm lucky, I might know where my starting point is, but from there, things can go anywhere. For example in my Hernandez series, I might have the protagonist Jorge head out the door in a rage, prepared to kill someone. However, what will he find when he gets there? What will interrupt him along the way?
Will his plans change?
Will he kill more than one person?
Will he be in danger?
Remember, as the author, you're with your character every step of the way. You're jumping in that vehicle and flying down the road. You're walking in that dark room and looking around. You're the one that discovers he forgot his gun at home.
I'm joking. He never forgets his gun at home.
Writing should be an adventure, not an overplanned itinerary. See where it takes you, and don't worry if it takes you somewhere you hadn't planned.