The Sherlock star reveals five secrets from the set of his new police drama

1 My character, Detective Supt Steve Fulcher, is attempting to bring the murderer of two young women to justice, but falling foul of the rules. This project had to be handled with extreme care – not only is it based on real life but it concerns police procedure. We had to reshoot an entire scene for legal reasons because a single word of dialogue had to be altered in the script. We had to be that precise!

2 Imelda Staunton and Siobhan Finneran play the mothers of the murdered young women, and there were a couple of very powerful scenes during filming when I almost lost it, when I thought they were going to shout ‘Cut’ and we’d have to go again, because I was so overwhelmed by the rawness of their grief. That level of distress made me forget I was in front of a camera and I almost gave in to emotion.

3 Smoking was a means by which Steve Fulcher engaged with the suspect [played by Joe Absolom, pictured with Martin], so it was crucial. But if you think I’m wincing when I puff a cigarette, that’s because I was. It’s horrible, one of the most unpleasant things you’re ever asked to do as an actor.

4 I swore like a trooper whenever I tried to get into Fulcher’s stylish Jaguar car and accidentally turned on the wipers or the indicators. It was great to look at but there was a definite lack of legroom.

5 Steve Fulcher was always at the end of a phone if I needed to consult him. So I’d text him and say, ‘How were you feeling on this particular day?’ or, ‘Are you left-handed or right-handed?’ It all helped to establish the character in my mind.