This is a fairly simple achievement and it must be done over the course of the Prologue, Episode 1 and Episode 2.
You'll notice that over every item / character / "interectable" object there's an exclamation mark when you rover the cursor over it, what you have to do is not "grab" / "push" it right away, just wait untill the marker converts into a "checked" mark, with that in mind just do that with at least 75 % of all objects / characters in all of the chapters.
As I said, fairly simple.
Edit: Even if you "observe" an item/person in one chpater, you'll have to do it again the next chapter. It only count for that specific chapter.
If you have any sugestions to make this solutions clearer just give a shout.
Edit 07/27/2015:
Altough some observations occur during the cut scenes, apparently it is possible to get 75% while skipping them. Thanks to TanukiV for the info.