But, with Unforgotten, that is far from the case.
The fourth series of the Bafta-winning ITV crime drama, which is hitting our screens this month, is as gripping as ever.
Written by Chris Lang, the six new episodes see DCI Cassie Stuart (Nicola Walker) and DI Sunny Khan (Sanjeev Bhaskar) taking on another emotionally-charged cold case murder.
This time, it’s the discovery of a dismembered body in a scrap metal yard, which the team believes has been stored in a domestic freezer for 30 years.
Here, we chat to Londoners Walker and Bhaskar to find out what is in store.
This series, Cassie is struggling with her personal life; her son has moved back home, and her father has early dementia.
Meanwhile, having retired from the police force for her own sanity and wellbeing, she makes the gut-wrenching decision to return – it turns out she is not entitled to her full pension payment unless she completes her 30 years of service.
One person who is certainly glad to have her back at work is her colleague and friend, Sunny.
“One of the things that has really struck me about people commenting on the show, from around the world, has been the warmth towards the Cassie and Sunny relationship,” notes 57-year old Bhaskar, who is also a comedian, having starred in sketch series Goodness Gracious Me and sitcom The Kumars At No. 42.
Meanwhile, in Sunny’s personal life, we see him finding more stability, as he moves in with his girlfriend.
“His home life has been quite chaotic as he put all of the organisation and care into his work life and very little into his personal life and we see him attempting to redress that imbalance this series.
“This time around there are elements within the story, which certainly were more emotionally challenging, for me, in a way they hadn’t been in previous series.”
An interesting and important element of Unforgotten is how it explores Cassie’s mental health struggles.
It is something viewers have been extremely invested in, last series the detective had what Walker, 50, says “would have been loosely termed a ‘breakdown’”, and it has been portrayed brilliantly.
How does Walker cope with this element of the role?
“I always think I am not affected by playing her – and it normally takes your family to tell you that are being [affected], by the way you are behaving towards them!” confides the actress, who is married to actor Barnaby Kay (they share a teenage son, Harry, together).
“If you are being affected, you want that to show on camera, you want that to show in the acting – not in real life.
“I feel huge, huge empathy for what Cassie is going through… You carry characters with you for the duration of the job because it is your job to look after the character, and there is a feeling of something coming off the shoulders when the job ends.”
When series 3 of Unforgotten aired in the summer of 2018, the team was immediately asked to make a fourth series, shares Lang.
But, during a lunch out he had with Walker and Bhaskar, it was decided it would be good to take a break to recharge before filming the show again so they did not start making series four until January 2020.
And then, 11 weeks into the shoot, lockdown happened.
It was not until September that they returned to finish filming, and Bhaskar says having six months off, and then coming back to set under such different conditions, was the biggest challenge he has faced whilst making Unforgotten.
Plenty of restrictions and guidelines were in place, so they were reassured they could all work safely, but the ongoing impact of Covid-19 was evident.
“You go and do your work, but when you come home, you’re still worried about the pandemic; you’re worried about your parents, your kids, or your family and friends,” follows father-of-one Bhaskar, who is married to actress and comedian Meera Syal.
“So, it’s a worry that sits there. And I think what Mainstreet [production company] did, which was remarkable, was that for the hours we were at work, we weren’t quite as worried about it there. And, you know, that was quite an achievement.”
Walker, who is also known for shows such as Spooks, The Split, and Last Tango In Halifax, admits she was “really, really nervous” about returning to the Unforgotten set after lockdown.
“But it was handled brilliantly and by the end of the first day I was thinking: ‘OK, they’ve gone above and beyond to ensure we are safe’.
“It was a huge relief and made me feel hopeful for the industry.”
Any addictive murder mystery will have plenty of plot twists thrown in – and Lang is particularly good at leaving viewers scratching their heads about how it will end.
Asked if she has to watch what she says when talking about the show, Walker quips: “You have no idea!
“There’s a sort of buzzing in my body, because there’s a part of me that thinks I’m going to go, ‘I’m going to tell you who the killer is!’ “It’s a really weird feeling where yes, you get a little bit worried that you might say something that would spoil anyone’s enjoyment of it.
“The worst ones are where they put you on the television, and it’s live, and I get this awful sort of nightmare feeling that I’m just going to blurt out…”
But one thing Walker and Bhaskar can happily openly discuss is guest stars.
We know that Sheila Hancock, Susan Lynch, Phaldut Sharma, Liz White, Andy Nyman, Clare Calbraith, and Lucy Speed are amongst the additions to the cast this year.
“It’s been one of the joys of each of these series, that you have these incredible people who come in – particularly in the interview scenes, where Nicola and I are just a few feet away from somebody giving a remarkable performance.
“It’s such a privilege,” enthuses Bhaskar.
“I remember saying to Nicola, ‘It’s like having front seats at the theatre, and watching someone just being brilliant’.”
Unforgotten returns to ITV on Monday February 22