Obvious authorial intrusion. When the characters start behaving like idiots or against their previously established characterization because the writer damn well needs them to in order to tell their story. May also occur when a character is accused of being used just to show a particular point of view, and not because they actually have it. The high-falutin' literary term for a character designed to express the author's preferred opinions (often the Only Sane Man) is the raisonneur--here at TV Tropes the preferred term is Author Avatar. A play on "Baby on Board".