I've seen two examples of actors rehearsing a scene, but in different accents to their own.
In Shaun of the Dead Simon Pegg and Peter Serafinowicz pretend to be John Lennon and Paul McCartney.
And in Star Trek, Chris Pine and Zachary Quinto rehearse in a German and Irish accent, respectively:
I imagine it's a way to let off steam or make a long day of filming entertaining, but is there a specific term for what's seen above?
For example, does the director say: "One more take, but this time let's do [term]", or is it more likely an idea that springs up on set?