Wolfenstein: The New Order

At the end of the first level, you will need to make a decision that slightly affects how the rest of the game plays out. One of the options will lead to saving Fergus, and thus this achievement; the other will net you
I would recommend going for this achievement (Fergus) in your first playthrough.

The differences between the timelines are lock-picking and armour upgrades if you save Wyatt, and hot-wiring and health upgrades if you choose Fergus. There are also a couple of different people to talk to in some of the bits between major levels.

The reason I suggest saving Fergus in your first playthrough is the health upgrades. On many levels you can get a permanent +10 bonus to your health; the equivalent on the Wyatt timeline just increases the amount of armour you get from each armour pickup, but the 100 total can't be increased, and armour is plentiful on most levels. There are 10 health upgrades over the various levels, which can thus give you a base health of 200.
This can carry over into your Wyatt timeline playthrough if you use the Chapters option instead of starting a New Game - selecting New Game will reset your progress on everything, including collectibles and perks, but you can use the Chapters option to jump into the alternative timeline with all perk and collectible progress retained.


EDIT: It would appear that the 'Wyatt Saved' achievement is glitching for some people. The achievement tracking on Xbox One is reading 1 of 1 requirement completed, but it is remaining locked for unknown reasons. I believe (and hope) that this will rectify itself in time, as the systems is clearly showing that the gamer has done what is required, and there have been numerous cases of X1 achievements unlocking days or weeks after the requirements have been met.
All the same, be aware that this has happened to some gamers, and consider saving Wyatt first, then immediately replaying the first level to save Fergus and then continuing that timeline for the benefits outlined above.

EDIT #2: This has now popped for everyone who I'm aware of it glitching for, so hopefully the issue has been fixed going forward.

At the end of the first mission, you will
For this achievement to unlock, Choose WYATT. Look at him and select him to

If this is your first playthrough, DO NOT pick restart level when the achievement unlocked or you will loose your progress and you will have to start over!!!

The moment your achievement unlocks, the checkpoint has been reached and reloading the checkpoint doesn't work either. You really have to play the first mission over again.

It is possible to save Fergus for the achievement and then open the pause menu before the checkpoint saves allowing you to select restart latest checkpoint and then choosing Wyatt for the other achievement.

You need to be quick to pause the game, usually as the camera is panning over BJ and the men but before the checkpoint is saved as you will have lost your chance. Make sure the achievement pops in the pause menu and if it doesnt wait a second or two more and keep trying before you get the checkpoint.

I recommend you saving the one you dont want to affect your story outcome first and then quickly restart checkpoint and save the one you do want. This saves you having to replay level 1 all the way through.