Martin Compston is once again joining the cast of Line of Duty to portray DS Steve Arnott, who gets promoted do DI in episode 2 (for a refresher on all the LOD acronyms and abbreviations, click here). His character was recruited to anticorruption during season one, after he refused to participate in a police cover-up following a bungled antiterrorism operation. Compston has also appeared in the 2018 film Mary Queen of Scots starring Margot Robbie.