There are several ways to accomplish this and I’ll detail a few depending on whether you’re playing Solo Theatrical Cut, Shared Story, or Solo Curator’s Cut. All of these involve making sure to pick only the head decisions as John as this is the best way to prevent him from believing in the supernatural.
Solo Theatrical:
- Head decisions only
- Have Andrew gets the gun and keep it.
- Make sure Angela survives her encounters with her demon and gets lost in the sewers.
- Don't even interact with the poppet in the museum.
- Once you get to the chapter “Low Point” and the group sees the “demon” in the distance, encourage Andrew to shoot. If done right, John should become rational at this point and the achievement will trigger.
- If you don't get it there, further into the chapter you'll play as Andrew and have to decide between helping Daniel or Taylor fight their demon. Select to help Daniel, but fail all the QTEs so that he dies. This will make it so you play as John later in spots you should've been Daniel.
- Once you get to "Ruined", upon being outside the factory, say to the group that you have to stop Mary and then select the "It might only be a theory, but it's all we've got" line as John. This line also provides a boost to the Rational trait that should unlock it if you didn't get enough previously.
Shared Story:
- Head decisions only
- Once you get to the museum where you play as John and Andrew, make sure the person playing as John select to leave the poppet alone rather than taking a look at it. This should be enough to make him rational and trigger the achievement.
Solo Curator’s Cut:
- Head decisions only
- Basically the same deal as above, except unlike the solo theatrical cut, you’ll be playing as John in the museum, so you can select to leave the poppet alone rather than avoiding it altogether.