Pharisee indicates "separatist," one who is separate from others in the world. About 200 years before Christ, the Pharisees seem to have arisen as a brotherhood with a sincere desire to resist the secularism into which Judaism had drifted. As the years went by, however, they added a great deal to God's written law and rejected counterbalancing commands also given in the Old Covenant—even such commands as those which appear in Leviticus 19, such as "You shall love your neighbor as yourself."
They then compounded this rejection with a vain sense of self-righteous superiority that in many cases excluded any contact with people of another ethnicity, and they even brought violence against other Jewish people who disagreed—as the apostle Paul's pre-conversion conduct shows. Paul was "a Pharisee of the Pharisees," and he went around throwing people into prison and, indeed, he may have even consented to the martyrdom of Stephen.
All this made these "separatists" very unattractive witnesses; they were in reality an embodiment of a rejection of God's intention of what a witness should be. Our witness does not have to bring about the conversion of others to be effective, because conversion is in God's hands anyway. However, it still has to be right, and being right in this requires personal sacrifices.
Being right—in addition to keeping God's commands—means being humble, modest, kindhearted, concerned, sympathetic, empathetic, helpful, warmhearted, friendly, gracious, serving, giving, charitable, open, hospitable, cordial, thoughtful, considerate, sensitive, cooperative, and on and on. One must do all of these things, while knowing full well that there is a line of separation across which we cannot allow ourselves to wander in our relationships with those as yet uncalled.
This is quite a plateful, but these are qualities that make God's way engaging and an attractive witness. God's involvement in our lives should give us freedom, as well as security against our fears of living and being this way. When combined with the keeping of God's commandments, it will produce quite a witness and go a long way toward keeping us clean—because we will be using God's Word as He intends.