In order of obtaining this achievement, you can't say a word during an entire playthrough.

What does that mean? This means you have to keep your fingers off the X, Y, B buttons. But keep in mind that there are some exceptions, so don't panic if your character talks sometimes.

- Some interactions with objects make Alex talk automatically, for example, opening some doors, using the radio transmitters or pushing the dumpster at the beginning. This is ok, the A button talking won't void the achievement.

- You can interact with optional objects with A, which will also trigger automatic dialogue (I did it a couple of times during my playthrough, and got the achievement anyways), but I recommend to go for this achievement once you have completed the game once or twice, that way you'll know what to do and will be able to just only interact using A with the objects needed to progress.

- Also, DugFreez has confirmed in another solution that there's an unavoidable dialog option at the end, when you have to pick Alex's future: "The game shows a final still picture and you have 3 dialog choices using the X, Y, and B buttons. I tried to wait it out, but you can not. At this point you have to press X, Y or B. Thankfully the achievement still unlocks...so that last use of X, Y or B does not count against the achievement".

So, the main rule is NEVER use X, Y or B dialog options. If, for some reason, you get distracted and talk, dashboard, press menu and exit the game. The game only saves when you enter an area, and probably pausing and going to the main menu would be enough, but do it this way, just in case.

Also, not talking in the entire playthrough will grant you the following achievements, if you don't have them already.

Worth 100 Gamerscore.