Spectator chooses one card and signs on it.
The selected card is placed in the middle of deck and the deck in to the box.

The magician puts a joker behind a box of cellophane and... the joker turns into a card chosen by the spectator!!!

The trick is almost impromptu.
One second of preparation and you have a trick you will be able to perform many times.

The Box can be given to the spectator before and after the trick.