My interpretation has always been and still is that he meant: "Don't let your enemies know that you truly think of them as enemies. Don't come out threatening or be blatantly irate with them. Keep you feelings or your thoughts of revenge to yourself. You always want to have the advantage by keeping them off guard. Always let them think that, you think of them as friends." That is, until the time is right or you can get close enough to them and just before you kill them, you tell them. Watch for the fear and know that you finally have justice.

The greatest example was killing Don Ciccio. Vito waited until the time was right. Came in as a "friend", quietly told him who he was and cut out his gut!