DI Matthew "Dot" Cottan

Dot Cottan is a detective sergeant in AC-12, the police anti-corruption unit, working alongside Kate Fleming and Steve Arnott. He is quiet, competent and unshowy, the one who handles the paperwork and the awkward errands without complaint. Colleagues barely notice him, which seems to suit him fine.

Craig Parkinson plays Cottan as furniture — pleasant, forgettable, always in the room but never the point of it — and that studied blankness is the whole trick. Line of Duty rewards close watching, and Cottan is the show's best argument for paying attention to the person nobody thinks to look at twice.