Following the RAW it seems impossible to surprise someone you are talking to with a sudden attack - because they can see you and the rules state you must be trying (successfully) to be stealthy to benefit from a surprise round.
I've taken this literally so far in my campaign - as soon as a player says 'I cast x spell' or whatever, I have everyone roll initiative, on the grounds that the other party can see the character in question starting to do something hostile, and a combat situation results.
Is this the way everyone is doing it? It seems to rule out the possibility of a sudden, unexpected strike from someone who is conversing with someone else. Maybe advantage on the Initiative check for the instigator, or something? Interested to know how others are dealing with this sort of thing.
Yes, when someone does something that would be a combat action, I have everyone roll intiative and we play it out - otherwise there would be no way other than my own fiat as DM to determine if Quickdraw McSpells got off their casting before Captain Dontwannagetshot had a chance to take cover.
As for attempts at trying to surprise someone who is looking right at you - I consider that to fall into the realm of "trying to be stealthy" and tend to use whatever seems appropriate for the situation at hand like a Dexterity (Stealth) check vs. Wisdom (Perception) to try and draw a weapon unnoticed or a Charisma (Deception) check vs. Wisdom (Insight) not to indicate through expression or body language that you are about to do something potentially hostile.
The most common skill contest, and the one mentioned in the PHB, is Perception/Stealth. But this is not the only possibility.
The DM determines surprise, RAW. But how he determines surprise is up to him. He can decide by fiat who is and isn't surprised.
However, skill contests are the usual way to resolve uncertain situations. Which two skills are used is up to the DM, and he should choose the two that make the most sense.
For creatures that are already visible to each other, where no violence is expected (like a market place or being introduced to the king), the aggressors Deception versus the Insight is the best pairing. The better the observers are at understanding body language, the more likely they are to divine violent intent. The more deceptive the aggressor, the more likely to catch the victims unaware.
There are other possibilities. A guy in plain sight but drawing a hidden blade? Use his Sleight of Hand instead of his Stealth or Deception.
Judgements like this are what DMs are for. 
ArialBlack wrote:
iserith wrote:
Thanks for the sensible answers but noone has really addressed my point on the RAW (perhaps it's best left for a 10-page debate in the rules forum)
ArialBlack wrote:
sim-h wrote:
Surprising someone you can see is impossible. The rules are pretty accurate on how to determine surprise so it's not really up to the DM. It explicitly states that it's perception vs stealth. It doesn't say "for example" or anything, it says this is how it is done.
So yes, sim-h, if you want to play according to the rules as written, your conclusions are correct.
Anything else is DMs houseruling. Whether houseruling is good or bad is debatable. I personally can't play with a DM that houserules, because I want a full overview on how the game plays before playing it. I don't want a DM to babysit me for the sake of "everyone having fun". But that's a whole different topic.
Rya.Reisender wrote:
I play it RAW. And the sentence you quoted is not stand alone. The follow-up sentences explains how the DM determines surprise.
Determine != Freely able to choose if.
Rya.Reisender wrote:
Actually if he's not paying attention it would be closer to the rules to give disadvantage on passive perception (-5) and still roll enemy stealth.
For ability checks against a DC the DM is allowed to determine it's trivial, but in a contest he does not have such priviledges.
Edit: The only extra priviledge I see the DM has is determining that noticing a threat is impossible for character on a certain rank.
Edit2: All rules about noticing threats and surprise talk about stealth vs. perception. There is no sentence in the rules that says that surprise can also happen in different ways. And it doesn't say anywhere "for example" which would indicate this is just one possible way to do it. It says "you do it like that".
Rya.Reisender wrote:
Rya.Reisender wrote:
Hmm, I'm not convinced. Remember that specific beats the general rule.
And your interpretation of the word determine is quite different than mine.
"The teacher determines what grade his pupils receive. To do this, he evaluates x, y and z."
For me, this doesn't mean that the teacher is allowed to freely give his pupils any grade he wants. For me, it means that he needs to try his best to evaluate x, y and z properly and give a grade he deems suitable according to these criteria.
Rya.Reisender wrote:
If you're wanting to have a nitpicky RAW discussion, I think this is well covered covered in Variant: Skills with Different Abilities, Player's Handbook, pg 175. Which suggests using skills with different ability checks. The DM doesn't even describe using skills tied to different ability as variant and describes it an action based on circumstance, Skills, Dungeon Masters Guide, pg 239. The ability for proficiency to be tied to different abilities is necessary for tool proficiency to function.