At night we'll lay in bed to sleep and just before I put earplugs in he says goodnight and I'll reply "Night, don't let the bed rats bite." and "nip" his belly with my hand
It started as a joke and it's become tradition now 
On the rare occasion that he has a lay-in, I take one of the [pet] rats in to wake him up when I think he's had a long enough lay-in.
If I pull my eyebrows up and top lip up to reveal my teeth DP breaks down into hysterical laughing, so I often do it when he's on the phone to his mother or on skype when gaming and put my face right next to his.
We occasionally start insulting each other for no reason, but it has to be creative insults. Thanks to Mumsnet I generally win 
We'll text each other something that makes no sense by using the predictive word function, so if you write a letter and see what word appears, then just keep clicking the next word that appears.
If the answer to something is "yes" then it's said "Yeeezz?"
This stems from a game we play where when we're on a car journey we have to attempt to make words from the three letters at the end of a reg plate.
When we're heading to bed for fun, as soon as one of us is naked the other says "Oh, did you trip and all of your clothes fell off?" Every bloody time. I really don't know how that happens
Usually it's DP saying it to me as somehow I'm always undressed first 
We also talk from the rats POV. They all have different ways of speaking.
I love 23-19 
Creepy Toby is brilliant, cackling at that!