A typical voiceover session.
Will Arnett has signed to play an unannounced character in the Michael Bay produced Teenage Mutant Ninja Turtles reboot. THR says he’s not playing Casey, and they add that the actors playing Splinter and Shredder haven’t been announced yet. Arnett joins a cast which includes Megan Fox as April O’Neil, Alan Ritchson as Raphael, Pete Ploszek as Leonardo, Noel Fisher as Michelangelo, Jeremy Howard as Donatello, and Jerry Mathers as the Beaver. (Leave it to Beaver jokes! Topical!)
We wouldn’t be surprised if Will Arnett were playing a coworker/adversary of April O’Neil (a douchey co-anchor, perhaps?). On the other hand, letting Will Arnett voice a sensei sewer rat would not be a huge mistake. A voiceover job of that magnitude should always be left to the professionals.