Scout gets surprised and killed in same turn. How to prevent that? Thanks.

could you be more specific?
Describe situation more detailed.

I'm at this underground base - I killed something like 20-30 enemies w/o taking any serious damage. Now my main character (scout) is sneaking around to clear all rooms. There's a small room with 2 toilets - it seems to be empty so my scout crouches forward slowly. All over sudden, there's an interrupt, an enemy spawns like 1 meter in front of me an pops my scout with a pistol. Game over. Playing on 2nd difficulty level.

There are these skills like Interruption (or something) that will probably help to get the first move when stumbling upon an enemy. I also noticed that speed seems to be important in some encounters; when you get interrupted, you might have 1 second to move into cover or shoot at the enemy - but that's not always the case.

Yes, there's 2 aspects:
1. Some NPC's is hard to detect, so they often initiate combat when it too late. I know of 3 such NPC.
Metagame solution - throw a heavy grenade/ use RPG/shoot trough wall in such locations.
"INITIATIVE" perk is very helpful in other cases - scout with it automatically moves first.

2. Interrupt mechanism in general - not always predictable when it'll happen. Read carefully ingame notes about it.
Line of sight is very important - if they see you first - they got you. Use 'moving while keeping direction' mode when go around corners and entering rooms.

I can't remember such cutscene. Was it german laboratory? Newhouse mansion?

Leaving combat mode - turn cloak mode on on every character, avoid detection and break visual contact with enemy, hit end turn cople of times.

Never go alone, use group tatics, or provide cover from snipers/machinegunners if your scout moves ahead alone. That helps in situations when enemy popup out of thin air. There's not much such enemies. Like i said there's 3 or 4 of them (Newhouse himself, guard in hallway in mansion, and those officer in "Organisation warehouse"). Remember those locations and use what i told you - grenades and fire at "empty" spot from safe distance enen if you don't see enemy there.

It's the "UK commandant's office" in Northern Britain. There are a bunch of armed civilians hanging around a house. As soon as you trigger the cutscene with the NPC, they turn into enemies and charge you. They implemented this by giving the enemy the first turn after the cutscene: an officer enters the room with the NPC, takes out his pistol and shoots your soldier to pieces. All you can do is sit there and watch it. You can't avoid being enganged in that room either, because you need to enter it to trigger the cutscene. The only way to manage this mission for me was to eliminate all armed civilians before I entered that room. In other words, I would hardly have a chance beating that mission on my first attempt, without knowing anything about it. To make matters worse, you can't leave combat mode most of the time during that mission.

Also, it's unclear to me how breaching a room is supposed to work in this game. Sometimes I open a door, discover an enemy, and get the first turn. In that case, there's usually a solution to the situation. If however the enemy get's the 1st turn, it's game over most of the time. Shooting through doors results in the same: door is gone, enemy get's first turn/interrupt, I'm dead. Throwing random grenades at undiscovered locations doesn't seem like a very efficient way.

All good games are incredibly frustrating, this one is no exception.

hmm... i playing this game since original release in early 2000s but didn't have THAT much trobles...
Am i correct it's Axis' campaign? Yes, storm through doors could lead to interruption in turn and things could get ugly.
If you know location or suspect that someone there or hear enemy in closed room - options:

1.Shoot through wall, not door in direction of enemy contact. Use rifles - they didn't create seethrough holes, so enemy cannot fire back. Not exactly fair method but helps in difficult fights - you can see blood spatter appear on floor even if you don't see enemy. That way you will know if you hit enemy.

2. While standing near door, but not in front of it shoot at door using SMG or MG couple of bursts, door will be destroyed (subvariant - place a grenade or mine on door using trapmode then shoot it - this could come in handy with smaller rooms or buldings - with heavy grenades like PWM-1 or RPG40 you can kill anyone in room in such fashion), then roll a grenade in room, one of the bright side of this game - characters smart enough to through a grenade around wall / corner without exposing themselves. Use round-shaped grenades - they can roll farther than other types.

3. Get to lower floor and shoot from beneath target. Or make another entry point by burst fire, then climb inside (that works with large indoor levels, not apllicable in UK commendature mission)

4. On higher levels lke 14-16, destroy door from safety, use scout, enter hide mode, crouching move from cover keeping direction for potential target - there's chance that you got them first.

Aaand there's plenty of tricks like that :))

Also. Scout as main character isn't a best choice: scouts most likely got hit or heavily injured. If non-player character got hit hard and got 0 and even slightly below HP will loose conciousness but remain alive, and you can carry him out of map and heal. If PC got around 0 - mission fail. Why devs made it this way not clear to me.

I have read about this, but it's unclear how grenades can be thrown around the corner. What I read in the forums is that you can toss them against a wall, and they bounce back into the direction of the enemy, maybe. But my grenadier can't throw a grenade into a room even if he stands right next to the door.

Also, no offence, but I can't imagine you haven't been blown to pieces again and again in this game on higher difficulty. Especially if you haven't played the map before and haven't memorized the position of enemies.

Regarding grenades, It's difficult to explain in text.
I suggest experiments in safe environment without enemy and ability to save/load, with different angles and distances from door and wall, avoid standing too close to walls in this - sometimes they blocking.

Ofc, i got into situation where i lost a man in such manner, in the past, until learned mechanincs (and broken parts of it) of a game.
Even now, it sometimes tricky - as i said couple of NPC is still a problem (especially one of them - that got to be captured alive, so half of methods aren't applicable).