One of XCOM 2's main features is the ability to spring an ambush on your opponents, giving you the perfect start to the first firefight. But given the right conditions, you could actually do this in XCOM: Enemy Unknown as well. Kind of.
In the video above I try to recreate the right conditions, but they aren't perfect so I'll apologise for that. You can get an idea of how to do it though. I did actually recreate the move several times in a different game, but that was on Ironman mode, so there's no save file I can load up. Sadpanda.
There are two elements to the new move in XCOM 2: practical and cosmetic. Practically, you can get a lot closer to the enemy than before, without them noticing you. As long as you don't step on the red "alarm" tiles, you're free to get into the perfect position (careful though, they go on patrol). Cosmetically, it's all about camera work, and XCOM 2 tries to artistically bring the camera around all of your ambushing troops as they fire.
Ideally in XCOM: Enemy Unknown, you'll have a couple of soldiers wearing ghost armour as you infiltrate the area around your enemy. Assaults will be able to get right close and still shoot, initiating the combat, while other nearby squaddies reveal themselves with overwatch shots. The first time the aliens notice you, either by a shot fired or something else, they'll go into a "scramble" mode, in which they run for cover. That's when all your overwatch units kick in.
This move also works very well with snipers upgraded with the skill to see whatever squadmates see, no matter how far away. It's one or two soldiers you don't have to move into position, and their overwatch shots are more reliable. I based pretty much the whole of my last Ironman run on this move, and it worked a treat every mission.
You don't even need the ghost armour, in fact. I've pulled it off before just by staying right on the edge of the enemy's line of sight. But this is very hard to reproduce.
Part of the magic in XCOM 2 is how the camera does a slow pan across all of your troops as they blast the enemy scrambling for cover. XCOM: Enemy Unknown does this in more of a disjointed way, focusing on one of your troops at a time. But I have had situations when all my troops were kind of in a line, and the camera acted very similarly to how it does in XCOM 2.
After having a few goes at reproducing the move here in the office though, I'm kind of glad XCOM 2 will just do all of this automatically. It's very nice to look at. Here's the nice, polished version in XCOM 2 at around 18:30 in the video. Mmm, can't wait for that.