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This is the only non story and missable achievement in the game. However, it's rather easy to get on your second playthrough. Just make sure to only click on the correct items/people with the correct inventory items(when needed), and not make a mistake. Clicking to walk around is fine. Just as long as The Projector doesn't shrug or shake his head, you're good.

You can also make sure to save throughout the game, so that if you accidentally do make a mistake, you can just reload without starting completely from scratch.

Seriously though, it's much easier than it sounds once you've played through once already.