The effect is generally worked as follows. Members of the audience write messages on small slips of paper, or billets. The messages can be questions for the deceased, or simply statements that the performer could not know. The billets are then put into individual sealed envelopes, which are collected and given to the performer. The performer then takes one envelope at a time and accurately describes the message inside. After announcing the contents of each envelope, the performer opens it, as if to confirm his reading.
The trick used in billet reading is the one-ahead method. It relies on the performer knowing what is inside one of the envelopes beforehand, and using that knowledge to stay one step ahead of the audience. He may do this by having a plant in the audience submit a pre-arranged message as one of the billets, or by secretly opening one envelope. When the performer pretends to read the contents of the first sealed envelope, he actually recites the plant's message or the message from the secretly opened envelope. When he opens the first envelope to "check" his answer, he actually reads the first billet, which he then pretends to "read" inside the second envelope. This process is repeated down to the final envelope, which is either an empty decoy, or the plant's envelope, or the secretly opened envelope. It is necessary, of course, that no one but the performer see the billets until the trick is completed and all the billets are out. If the secret-opening variant is used, the performer must use sleight of hand to conceal that the last envelope is already open, or to "extract" the last billet from an empty decoy envelope.
