Surprise

When characters are surprised, attackers automatically take their first action before the surprised side can act. The surprised side cannot roll defensive skills to dodge or parry that first action.

Determining Surprise (Ambushes)

Each attacker makes a sneak roll. When targets come within range, each target rolls search or Perception a couple seconds before the ambush is sprung. If a target's roll is equal to or higher than any attacker's sneak roll, they spot that attacker and are not surprised. If none of the targets spot the attackers, the targets are surprised. (Ambushes are a great place to use "Secret Checks" as discussed in "The Rules.")

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Source: REUP