Audio commentaries in which the participants are characters from the thing on which they're commentating.
Additionally, some TV series examples:
* The King of the Hill DVDs feature in-character commentaries on certain episodes.
* I'm Alan Partridge series 1 DVD includes commentary from Alan and Lynn.
* The 2004 Rob Brydon series Director's Commentary was entirely based around the gimmick of a fictional director commentating on episodes he'd directed.
