Filling your notebook is matter of meeting all people throughout your adventures in Fortune City. This includes psychopaths, survivors and storyline characters you meet. Most of these will come naturally by playing through the game, while other entries require some effort and timing.
Although this achievement is possible in one playthrough, it's certainly not a requirement, if you happen to miss some, you'll have a chance of completing them in subsequent playthroughs.
This achievement is best combined with "Hero of Fortune City" A walkthrough for gaining both in the same playthrough can be found here.