One of the many aspects that you could refer to in answering this question would be the way in which Shaffer organises time in his play in a rather novel and unique way, which allows him to focus on the ponderings of Dysart as he begins to question the boundary lines between sanity and insanity, and to what extent Alan is actually insane or not.
The play begins in the present, and as Alan's treatment begins and continues, we are shown a series of flashbacks to Alan's childhood and the night of the "crime" and also Dysart's life before he begins treating Alan. However, at times, past and present are shown to intermingle in rather startling ways, when one topic or event is talked about simultaneously in both past and present. This occurs for example when the Nurse and Dysart talk about Alan and then at the same time Dysart tells Hesther about this conversation.
Such a fluid treatment of time and the way that two different time periods can be juxtaposed so effectively allow Shaffer to focus on the way in which Alan's story is forcing Dysart to question his own notions of sanity and insanity, and greatly strengthen his reflections on whether these notions exist independently, or are just flexible, ever-changing creations of society at any given time. Just as Shaffer plays with time to allow us to see different perspectives on the action, so sanity and insanity are shown to be ever-changing products depending on the time and context that we are refering to. Temporal fluidity is key on so many levels.