Three things must be clearly understood: karma (right action), vikarma (forbidden action), and akarma (inaction). The path of karma is deep — like undercurrents beneath a river's surface that are invisible from above. Without careful attention, one can easily confuse one for another.
Krishna is being honest with Arjuna. Before giving the answer, He wants the student to respect the difficulty of the question. Rushing to conclusions about what is action and what is inaction leads people astray. The first step toward understanding is admitting the depth of the subject.