As you play through Detective Pikachu, you will earn what are known as Pika Prompts. These are Little video cutscenes where Pikachu will communicate with Tim about the Case at hand or even some General topic like Coffee, and there are loads of them to obtain in every single chapter of the game. (After obtaining a Pika Prompt, it will be properly stored without needing to be activated again.)
In order to obtain all the Pika Prompts, you will need to say alert while playing, talking with Pikachu whenever you are prompted to on your lower screen. On top of this, you will need to talk with Pikachu periodically out of the blue, as this will see you unlock a number of the General Pika Prompts that don't have anything to do with the chapter you are in. Furthermore, you can obtain every last Pika Prompt simply by scanning the special Detective Pikachu Amiibo while playing.