Working on my second novel, something strange happened that I hadn't accounted for in my chapter plan. The villain insisted on being seen (well, one of the villains). I initially had planned to hint at him and his cohorts but no, he wasn't happy with that.
Steven Pressfield's post, The Second Act Belongs to the Villain is about just this topic, keeping "the antagonist front-and-center in the middle of your story". Have a look.