Reacher star Sonya Cassidy has admitted she was a little apprehensive taking on the role of Susan Duffy in season three for one particular reason.

The DEA agent from Boston teams up with Alan Ritchson's nomadic ex-Military Policeman in season three to infiltrate a crime ring suspected of abducting one of her colleagues.

Three episodes premiered on Prime Video last week and the exhilarating case, based on Lee Child's 2003 novel Persuader, will Thursdays through to March.

"[Duffy] is someone who's quite similar to Reacher in many ways," Cassidy explained during an exclusive interview with Screen Time.

"This show really hits the ground running, Duffy is already in way up to her neck. So she's kind of sprinting ahead, trying to put out a load of fires.

"She doesn't have time to figure out who this guy is, that kind of happens steadily over the season. But there is an immediate kinship there of knowing s*** has hit the fan, I need you to be on board with me and help me fix it, or get out of my way.

"And that's really refreshing to see, and it's nice to see those similarities. But, at the same time, we also see Duffy trying to figure him out and a trust and respect grows over the season."

Having teamed up with young cop Roscoe Conklin (played by Willa Fitzgerald) and his Army pals Karla Dixon (Serinda Swan) and Frances Neagley (Maria Sten) in previous seasons, Duffy's tough, no-nonsense approach is a refreshing change of pace.

However, portraying Duffy presented its fair share of challenges for Cassidy, not least of which was the fact the British star was born thousands of miles from her character's hometown.

"I've always loved the challenge of accents," she shared. "I find them fascinating. It's so much more than sounds, it's a whole psychology and culture and way of being and I think the more regional accents we hear the better.

"But I was nervous taking this one on! I've done a lot of American in my time, but Boston was always going to be a challenge."

Thankfully, Cassidy was well-prepared right from the jump with extensive research before she had even gotten started with filming.

"For the audition I just watched a lot of films set in Boston," she recalled. "I found this gorgeous clip from the 80s of these Boston longshoremen talking about working on the docks and family life, which was great and kind of became the soundtrack to life on set to get my head in gear.

"But I did also have a dialect coach who helped me hone it.

"I always try and get it as perfect as possible, I never want the accent to distract from the character and the show as a whole, because when it's not good it is quite distracting."

Fans can see how Reacher and Duffy's partnership develops throughout another tension-filled outing for the hit thriller over the coming weeks.

Reacher season 3 continues Thursdays on Prime Video.