Worst Fears

While controlling another player, you can see all cards in the game that player can see. This includes cards in that player's hand, face-down cards that player controls, and any cards in that player's library the player may look at.

Controlling a player doesn't allow you to look at that player's sideboard. If an effect instructs that player to choose a card from outside the game, you can't have that player choose any card.

You can use only the affected player's resources (cards, mana, and so on) to pay costs for that player; you can't use your own. Similarly, you can use the affected player's resources only to pay that player's costs; you can't spend them on your costs.

You can't make the affected player concede. That player may choose to concede at any time, even while you're controlling them.

You could gain control of yourself using Worst Fears, but unless you do so to overwrite someone else's player-controlling effect, this doesn't do anything.

While controlling another player, you make all choices and decisions that player is allowed to make or is told to make during that turn. This includes choices about what spells to cast or what abilities to activate, as well as any decisions called for by triggered abilities or for any other reason.