A hero may choose to wait and see what happens by taking a Hold action. He may then go later in the round if he chooses. A Held action lasts until it's used. If a character has a Held card when a new round starts, he's not dealt in.

If a character is Shaken while on Hold, he immediately loses his “Hold” status and any remaining actions for the round. If the victim soaks the attack and avoids the Shaken result, he remains on Hold.

Interrupting Actions: If a character on Hold wants to interrupt an action (including a rival who was also on Hold), he and the opponent make opposed Agility rolls. Whoever rolls highest goes first. In the rare case of a tie, the actions are simultaneous.

Savage Worlds Deluxe Edition p. 72