The former My Family star has taken over from Ben Miller as the lead detective in this hugely successful series - which means he now gets to spend six months of the year filming in tropical Guadeloupe.
But while some actors may have celebrated the news that they’d landed the role of Saint-Marie’s newest sleuth, DI Humphrey Goodman, by cracking open the champagne while browsing online for shorts and flip-flops, Marshall admits to feeling nervous.
“It was daunting to begin with because I have never joined an existing show and cast before and as it’s such a popular show, you don’t want to mess it up.
“I was really honoured and humbled by the BBC entrusting me with one of their most popular shows. That really meant a lot to me. I just wanted to assimilate myself but still keep the series familiar to viewers... I want people to like it as much as they did before.”
It would be understandable if his character was also experiencing doubts about his own new job - after all, last week viewers saw him arrive on the island to investigate the death of his predecessor Richard Poole (Miller), which might have made a lesser cop think twice about staying on.
And then there’s the small matter of his new colleagues.
Marshall explains: “To begin with... they think he is quite odd, a bit of an idiot and in the way. Of course, they are grieving the loss of their previous boss and don’t react well to some guy from London coming in and literally treading all over their investigation.
“However, gradually, as the series goes on, he begins to fit in as their little odd boss.”
Hopefully, Humphrey will start to win them over this week, as the team investigate a murder on the set of a zombie movie.
The victim is a stand-in, who appears to have been poisoned - but what motive could anyone have for bumping off someone with such a lowly position in the movie’s pecking order, who had only been on the island for a week?
The cops begin to suspect the murderer got the wrong woman, and the real target was the film’s star, Lexi Cunningham (Michelle Ryan). Three key suspects emerge in the shape of director Carl Collins (Rhys Thomas), scriptwriter Arnold Finch (Peter Davison) and on-set runner Susie Jenkins (Hannah Tointon), but are any of them really capable of murder?
It may be a tough case for Humphrey, but at least it was fun for Marshall.
He laughs: “One of my favourite people to work with though was Rhys Thomas. We are about the same age, both have kids and one night did a karaoke off! “We sang Under Pressure - he sang the Freddie Mercury bit and I sang the David Bowie bit, and it was awesome!”
Maybe that new job isn’t quite so daunting after all...