Thomas Watson, A Divine Cordial, Chapter 7, Section 4:
The Lord does not tie Himself to a particular way, or use the same order with all. He comes sometimes in a still small voice. Such as have had godly parents, and have sat under the warm sunshine of religious education, often do not know how or when they were called. The Lord did secretly and gradually instil grace into their hearts, as the dew falls unnoticed in drops. They know by the heavenly effects that they are called, but the time or manner they know not. The hand moves on the clock, but they do not perceive when it moves.
Thus God deals with some. Others are more stubborn and knotty sinners, and God comes to them in a rough wind. He uses more wedges of the law to break their hearts; He deeply humbles them, and shows them they are damned without Christ. Then having ploughed up the fallow ground of their hearts by humiliation, He sows the seed of consolation. He presents Christ and mercy to them, and draws their wills, not only to accept Christ, but passionately to desire, and faithfully to rest upon Him. Thus He wrought upon Paul, and called him from a persecutor to a preacher. This call, though it is more visible than the other, yet is not more real. God’s method in calling sinners may vary, but the effect is still the same.
Source: http://www.puritanboard.com/showthread.php/88551-Watson-on-conversion, Comment #1