When Nathan shows aggressivity towards Kyoko is an important scene that will have and impact the rest of the movie, because it is really the moment when Caleb becomes suspicious of him. Indeed, Ava told Caleb that he sould not trust Nathan, and when Caleb saw him being aggressive and losing his calm for no good reason, he starts questionning if Ava is right. It's the moment when Nathan truly begins to lose Caleb's trust.

When Nathan brings Caleb to his lab and explains to him the running of Ava's mind is also an important event. He tells him that he built a self-evolving brain which makes Caleb being even more attached to Ava. In this scene, Nathan makes Caleb realise that if Ava flirts with him, it's not because she is programmed to, but because she really has an interest for him. Caleb really gets emotionnaly involved as of this moment.

Nathan tears up Ava's drawing is the absolute moment when Caleb starts doubting the hole experiment. Not only he feels bad and sad for Ava, who showed him her drawing earlier in the day, but he now begins to see Nathan as a threat, a dangerous person. All of this happens because Nathan's ripping of the drawing made him seem angry, mean and untrustworthy. In reality, Nathan knew he was being watched and use this scene as diversion to hide a camera in the room.

One of the most important scenes is the one when Nathan gets so drunk that he passes out. At this moment, he gave Caleb the opportunity to steal his key card and to do some investigation of his own. Because Nathan loses all of his capacities, Caleb sees a footage of him building various AIs, he discovers Kyoko is a robot, he speaks to Ava and tells her his plan of helping her escape and finally, as we learn later in the movie, he reprograms the entire facility to open all the doors during the next power cut and then to lock the doors. It's the scene that changes the rest of the story dramatically.