The magician presents to the spectator a deck of blue backed cards and shows that this is just a regular deck of 52 cards. The magician places the deck, face up, on a table. This is the last time that the magician will touch the deck.
The magician invites the spectator to cut the deck approximately in half and place it next to the original pile. The spectator is then invited to select one of the two piles which is then put to one side.
From the selected pile the magician invites the spectator to deal the cards face-up and to stop at ANY card they wish. This gives them a genuinely free choice.
The selected card is placed away from the rest of the deck. The magician now takes the remaining cards and specifies that truly, the spectator had a free choice and the card they chose isn’t a duplicate of any of the other blue backed cards.
However, on turning over the spectator’s selected card the spectator is GOBSMACKED to see that not only is the card a red backed card, it also has their name written on the back (or any message you want).
They suddenly feel that perhaps their free will isn’t as free as they thought!
[Note that you will need two decks with different backs or back colors, for example a red backed and blue backed deck, to perform this effect.]