Meet the cast of Line of Duty series four

Trusty, albeit cocky, copper Steve Arnott is one of AC-12's best assets. Honest and hardworking, he's an expert in turning the screw on bent coppers – although his eye has a tendency to be turned by a pretty face. He also found himself in hot water last series when corrupt Matthew "Dot" Cottan attempted to frame him as the Caddy – a turn of events that saw Steve arrested, losing his fiancé and the trust of most of his co-workers... except loyal AC-12 colleague Kate Fleming.

Where have I seen Martin Compston before?

A Scottish native, Compston keeps his south London accent for the entire Line of Duty shoot, even speaking in it when he's not filming. On the road to becoming a professional footballer, he was cast as an unknown by Ken Loach, appearing in his 2002 film Sweet Sixteen before going on to land parts in Monarch of the Glen, Red Road, A Guide to Recognising Your Saints and The Ice Cream Girls. Last year he played serial killer Peter Manuel in ITV drama In Plain Sight.

DS Kate Fleming (played by Vicky McClure)

Trusty Kate Fleming was promoted at the end of the last series for her brave attempts to apprehend Dot – securing his dying confession and ensuring that former Chief Superintendent Patrick Fairbank was sent behind bars. She remains at the heart of AC-12 for series four – but there's a spot of tension with colleague Steve Arnott now they both share the same job title.

Where have I seen Vicky McClure before?

McClure first came to attention as Lol in Shane Meadows' This Is England in 2006, going on to star in follow-ups This Is England '86 (for which she won a BAFTA), This Is England '88 and This is England '90. She's also had a role in the first two series of Broadchurch, plus film appearances in Hummingbird, Svengali and Madonna's directorial debut Filth and Wisdom. Most recently she's starred in BBC1 drama The Replacement.

Superintendent Ted Hastings (Adrian Dunbar)

Firm but fair Ted Hastings has one thing on his mind: identifying and punishing corrupt coppers. As AC-12's Senior Investigating Officer, he personally recruited Arnott and Fleming, but the pair came to doubt him in series three when they discovered he was a Mason.

Where have I seen Adrian Dunbar before?

A veteran of TV, film and theatre, Dunbar has appeared in a string of projects including My Left Foot, the Crying Game and The General. He showed up in the very first episode of Cracker and the last of A Touch of Frost and most recently played a part in BBC's Shakespeare adaptation The Hollow Crown. Active behind the scenes, he also earned a BAFTA nomination for Best Original Screenplay for his film Hear My Song.