In a Hunger Games-like fighting pit, a character knows he is fighting a rogue that will try to ambush him. This area is densely forested, but small, so the encounter will occur very soon.
If he is on high alert the entire time, would he still be surprised?
If he sees the Rogue coming, he's not surprised. If he doesn't see the Rogue coming, he's surprised. From the PHB, page 189:
Any character or monster that doesn’t notice a threat is surprised at the start of the encounter.
So it's not about readiness, it's about what you perceive. This is important, because players will often declare that they are on high readiness, or say that they always expect to be attacked. Unless you're happy with the player characters never being surprised, it's best to disallow this (unless the PC has the Alert feat).
As @MatthieuM points out, if you want to represent a state of active readiness, a good way to do this is to make Perception checks actively rather than just relying on passive Perception to see enemies coming. So if your player complains that his readiness isn't doing anything, this could be a good compromise.
Initiating a fight while being unnoticed should give an upper hand. You can't negate it just by saying "I was on high alert", without investing anything. Potential investments could be the Alert feat, the Perception proficiency, an Inspiration point being spent on the Perception roll, etc.
Chapter 8: Adventuring has the "Noticing threats" paragraph:
Use the passive Wisdom (Perception) scores of the characters to determine whether anyone in the group notices a hidden threat.
While traveling at a fast pace. characters take a -5 penalty to their passive Wisdom (Perception) scores to notice hidden threats.
This implies characters are always searching for threats, by default, players don't have to announce "I'm on high alert" every time for this. You even can try to be stealthy while moving at a slow pace (and you don't have to be a rogue for that). That probably means the rogue itself can also be ambushed.
Surprised mechanic models two different things in 5e:
Unless Surprised is meant to trigger a feature (like rogue's Assassinate), you might just roll for initiative and let things happen naturally.
The final result depends on the situation:
Situation 1. The combatants are in place, both are aware of each other presence, and combat is imminent. This is already a combat encounter. Ask players to roll for initiative.
Situation 2. The character doesn't know, how the rogue attacks. Is he/she alone? Will they try to backstab? Will they shoot from afar? The character isn't sure if the rogue is still around. It is a normal "travelling in a hostile environment" situation — use the passive perception check.
PH doesn't say much about how surprise is determined, leaves it to the DM:
If the adventurers encounter a hostile creature or group, the DM determines whether the adventurers or their foes might be surprised when combat erupts.
DMG lists surprise as a drawback:
Modifying Encounter Difficulty
Situational drawbacks include the following:
- The whole party is surprised, and the enemy isn't
It doesn't give clear instructions either, and lets the DM use common sense instead.
Surpise (phb 189):
The DM determines who might be surprised.
Whether surprise actually happens depends on the opposed Stealth and Perception checks.
An ambush can be expected, the defender is very actively looking, so I would give advantage to the Perception roll. Then let the dice decide.
Just have them role play their security measures, then see if the rogue actually trips these. If they do not specifically state they are looking around for enemies, or setting up traps that the enemy will trip (trigger) during their approach (you decide their approach), then surprise them if their perception is not high enough. Use passive perception for normal expectations, normal perception checks for active searching.
For example, if the character is scanning the visible horizon and the rogue silently drops from a tree, they will be surprised unless they have abilities such as the alert feat.
No. By the same principle (and rules) that says you cannot be surprised once combat has begun, even if new creatures enter the combat, I would rule in this specific case that combat is specifically expected to be imminent, and initiative should be rolled upon entering the arena, so no surprise unless one party was already in the arena waiting in ambush when the other entered.
