One can hardly picture a modern sleuth delaying long in an attempt to evangelize his quarry, but these general principles are the right stuff and shine like good deeds in a naughty world.
As one peruses this little pink pamphlet he is constantly struck by the repeated references to the detective as an actor.

That was undoubtedly the ancient concept of a sleuth.

"He must possess, also, the player's faculty of assuming any character that his case may require, and of acting it out to the life with an ease and naturalness which shall not be questioned." This somewhat large order is, to our relief, qualified a little later on.

"It is not to be expected, however," the author admits, "that every detective shall possess these rare qualifications, although the more talented and versatile he is, the higher will be the sphere of operation which he will command." The modern detective agency is conducted on business principles and does not look for histrionic talent or general versatility.

As one of the heads of a prominent agency said to me the other day: "When we want a detective to take the part of a plumber we get a plumber, and when we need one to act as a boiler-maker we go out and get a real one--if we haven't one on our pay rolls." "But," I replied, "when you need a man to go into a private family and pretend to be an English clergyman, or a French viscount, or a brilliant man of the world--who do you send ?" The "head" smiled.
"The case hasn't arisen yet," said he.