There are two ways this can go: cute and fluffy or dark, either way quite cracky.

Cute and fluffy option: Newt keeps notes on himself the same way he does for his creatures, including details of his relationship and his mate's attempts to impress him (and their successes)

Dark (TW: Non-con): Grindlegraves forces Newt (via threats or a spell) to talk as he rapes him. He expects awkwardness. What he gets is a clinical, Attenborough style narration of what is happening.