Tackling Colin Dexter’s fictional hero, Endeavour Morse, has given Shaun Evans a platform from which to flex his acting muscles, portraying a younger version of the melancholic, hard-drinking, irascible Morse that audiences grew to love when the late John Thaw played him from 1987 to 2000.

The previous series of Endeavour saw the young detective’s father die and Morse shot and injured. In the second series it’s four months on and Morse has been transferred to a different station, removing him from the kindly influence of DI Fred Thursday (Roger Allam).

“When we come back this time, Morse is isolated and unhappy and not firing on all cylinders. A lot of that is to do with him drinking too much,” says Shaun. “You get to see Morse at home, so you understand why he’s behaving in a way that makes the rest of the office go, ‘What’s wrong with you, man?’

“Morse has inherited some of his father’s bills, so there’s financial pressure, and he has to look after his stepmum and sister. He’s in a slightly darker place.”

For Shaun, 34, it’s a new experience to return to a role, and he’s keen that neither he nor the series appear to be coasting on their success.

“My feeling is, if you’re coming back to do it again, you need to up your game,” Sean told the Daily Express. “You have to do the things you felt you didn’t achieve last time.”

It’s an admirable sentiment from the actor, who’s had roles in Whitechapel, The Last Weekend and Silk, among others.

While Shaun brings youthful energy to his role, Roger Allam, 60, brings a more measured wisdom as Morse’s mentor, Thursday. Roger, also known from BBC2’s political satire, The Thick Of It, says Thursday is concerned about Morse’s mental state.

“He’s concerned that Morse is drinking a bit too much and that it’s a sign that not all is well,” he says.

Roger appeared in a 1996 episode of Morse and is an acclaimed theatre actor who has largely stayed away from TV to avoid typecasting. “One of the things that attracted me to Fred Thursday is that he’s a character I’ve not played much,” he says. “He’s down to earth, a good, pipe-smoking individual. I’d love to be in a western, and probably playing Thursday is the closest I’ll get to it, with the occasional bit of gunplay and fisticuffs.”

Roger, who grew up in London’s East End, has a personal reason for wanting to play Thursday, who fought in the Second World War before becoming a policeman.

“My father was a vicar in the East End just after the war,” he says. “And war was still very present as I was growing up. One of the reasons I love playing Fred Thursday is that it’s a slight way of honouring that past and narrative and what people lived through.”

The new series sees Morse tackling murders against the background of the 1966 World Cup and Bonfire Night, hunting a fictitious Oxford Strangler and investigating a boys’ borstal and a private girls’ school.

Endeavour returns on ITV1, Sunday, March 30