Liquidsabre said:
Right, no readied actions outside of combat. PHB p. 137 (The Surprise Round):
"Any combatants aware of the opponents can act in the surprise round, so they roll for initiative."
Either way the sniper rolls inititative and takes their surprise round, in this case, readying an action. Or firing if one of the PCs spotted him and decides to attack I suppose.
That there is a surprise round already presumes that combat has begun. Put another way, you cannot ready an action outside of combat.
Here's how I read the situation:
It's high noon, there's not a cloud in the sky. Abe and Bill have come to settle their differences once and for all. Snape hears word of it, and waits on the roof of a nearby building for reasons of his own.
Abe slowly saunters down the dusty street. A tumbleweed lazily crosses in front of him, as he wipes the sweat off his brow. On either side of the street, townsfolk are quickly scurrying into buildings, shuttering windows, and otherwise making themselves scarce. Snape watches as Bill steps into view.
Abe: "I've had enough of you bad-mouthing my horse! You're gonna 'pologize to Betsy or I'm gonna put you down right where you stand!" Abe's hand hovers over his six-shooter, his fingers flexing in anticipation.
Bill: "Well, you know Betsy was my horse 'fore you stole her from me. Y'ouse nuthin' but a filthy horse-rustler." He spits before adding, "Yeah, we's gonna settle this all right, we's gonna settle this good!"
What happens next depends on whether Abe or Bill notice Snape. Spot checks all around versus Snape's Hide roll. If Snape wins, he can either watch, or initiate combat. If Abe or Bill spot Snape, then what they do about it plays a big role in the outcome.
If Snape successfully hides and continues to watch, that is, he does not initiate the combat, then when gunfire does erupt, he rolls initiative along with Abe and Bill. When his turn comes around, he can chose to do whatever he wants, including continuing to watch.
Five minutes tick by as Bill and Abe cuss each other out. Townsfolk peek anxiously through cracks in the doorway, wondering when the show's going to begin. Suddenly, Bill reaches for his pistol...
Roll for initiative. Bill has initiated combat, but because of luck, Dex, and Improved Initiative, Abe goes first, then Bill, and finally Snape.
...but Abe is a split hair faster, shooting and moving to one side. With his left hand, Bill clutches his chest. He looks down, surprised to see the crimson flower growing there. His head tilts up one last time to look his killer in the eye, and then he collapses face-first into the dirt.
"Horse-pucky," thinks Snape from his perch, surprised at how quickly violence erupted. "Bill was my friend, I ain't gonna stand for that!" He fires at Abe.
Since Abe hasn't spotted Snape, Snape is effectively invisible to Abe. He gets the +2 to hit, along with another +1 for attacking from higher ground, and Abe, who doesn't have Uncanny Dodge, loses his Dex bonus to AC for the shot. (Had Snape been visible, off to one side, but not regarded as a threat, Abe would instead simply be flat-footed to Snape's dirty, underhanded, surprise attack.) Snape miraculously misses. And that is the extent of Snape's "surprise round", which isn't one in the sense of a free attack, but rather in the SURPRISE! aspect of the action he is allowed to take.
Since Abe is next in the initiative order, he goes next. Abe sees Snape up on the roof of the bank, and starts firing back while dashing to the other side of the road, looking to find some better cover. And combat continues normally from there, as you would expect.
Now, had Snape initiated combat:
Five minutes tick by as Bill and Abe cuss each other out. Townsfolk peek anxiously through cracks in the doorway, wondering when the show's going to begin. Snape thinks to himself, "Are these two yahoos gonna yammer on all day? Abe? It was nice knowing ya..." He lines up his Springfield and fires.
Snape has initiated combat. Everyone rolls for initiative. In the order rolled, Abe goes first, then Bill, followed by Snape. But since neither Abe nor Bill are aware of Snape, Snape gets his surprise round.
A *CRACK* thunders from the rooftop behind Abe, and a little puff of dirt erupts next to his right foot.
Since Abe hasn't spotted Snape, Snape is effectively invisible to Abe. He gets the +2 to hit, along with another +1 for attacking from higher ground, and Abe, who doesn't have Uncanny Dodge, loses his Dex bonus to AC for the shot. (Had Snape been visible, off to one side, but not regarded as a threat, Abe would instead simply be flat-footed to Snape's dirty, underhanded, surprise attack.) Snape miraculously misses. And that is the extent of Snape's surprise round. Abe goes next, as he's still first in the initiative order, and combat continues as you would expect.
If Snape had initiated combat and won the initiative, he would get to act twice before anyone else gets the chance to react, once for his surprise round, and again for being on top of the order.
A *CRACK* thunders from the rooftop behind Abe, and a little puff of dirt erupts next to the surprised cowboy's right foot. Abe spins, trying to spot the sharpshooter, but Snape has already snapped back the bolt and... *CRACK*
Bill stares, dumbfounded, through the space where Abe's head used to be. His hand moves away from his pistol as his thoughts tries to catch up to his heart, lodged somewhere upwards of his throat.
This is why rogues find Improved Initiative to actually be a decent feat.
Now, if Abe spotted Snape and Snape knew he was made, but Bill failed his own Spot check, then both Abe and Snape would get to act in the surprise round, while Bill simply stands there trying to think of an appropriate comeback to Abe's empty threats.
The point to remember is, even though somebody may think they know what's going on, they can still be surprised. And sometimes, because of indecision or bad rolls, even though by all rights one party should be going first, you can't always count on it.
Cheers,
Vurt