Warmaster Horus
Explorer
I'd even consider the guards to have readied attack actions.
If the guards have noticed any member of the party before the barbarian attacks they are not surprised when he does.
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However the scenario is even sillier when you get say an Assassin rogue into the mix.
Assassin has sharpshooter feat, and is hidden 600 feet away from a lone sentry.
Assassin "I attack him"
INITIATIVE
Guard rolls high.
Assassin says, eh I don't attack if I won't get my assassinate ability.
Do you continue in initiative for the next hour the assassin watches the sentry? Or does he get to attempt again after a round, 10 rounds? Keep in mind nothing has happened from the perspective of the sentry.
Like I said, in the right situation. That example would not be one.So a heavily armed man is granted an audience with the president. He seems nice enough. The Secret Service would be caught completely off-guard if he suddenly attacked.
Surrrrre.
Guards guard things by being on guard. In the absence of some special circumstances it'd be impossible to surprise the guards.
Also, any competent guards would have disarmed all the PCs and suspected spellcasters would have their foci and spell components taken. A Detect Magic spell would be cast and all magic items removed. If the PCs wished to keep a magic item an identify spell would be cast to ensure the item was not harmful.