Unforgotten's Sanjeev Bhaskar teases a backpack surprise for Sunny in season 4

Speaking to RadioTimes.com and other press ahead of season four, Bhaskar confirmed that the backpack would be returning once more, before teasing its mysterious new contents picked out by the costume department...

"At the very beginning it was just a functional bag," he explained. "It is just a bag, and it suddenly became this thing, and I think it was the second series I thought, 'Oh well we could have fun with this. We'll get the costume [and] props department just to stuff it with things and then it'll be fun for me to see what they put in there'. And then it became this thing.

"We did continue that on series four, so yeah, I've definitely taken pictures of the contents that they put in and I've very dutifully - people may or may not believe it - but I very dutifully did not open the bag until the end of the filming day, so I had no idea what I'd been carrying around!"

He also teased that season four had marked the first time that he had something "that moves" hidden inside the backpack.

"There was a point early on where I said to costume [department], 'You can go wilder than this by the way. You can put anything you want in there. Let your imagination run riot'. And they did. The only thing I will tell you as a spoiler is it's the first time they gave me something - an object that moves."

Season four will see Sunny return opposite DCI Cassie Stuart played by the show's co-lead Nicola Walker. However, at the start of the series Cassie is absent from the show's central cold case investigation, as she's petitioning to retire from the police force early.

Bhaskar described Walker's absence as "challenging," explaining: "I think that there is more of this series which is, you know, not having Cassie having present, and that was more challenging, because I've got so used to having Cassie around... One of the character's challenges and for me personally was not having Cassie there or Nicola there."

The series also faced major filming disruptions due to lockdown restrictions. Speaking about the impact COVID had on production, Bhaskar said: " [To] have six months off and to then come back under such different conditions, I think that was the biggest - I can't think of a bigger challenge that I've had than that across the series."

Unforgotten season four will air in 2021. Looking for something to watch? Visit our TV Guide or find out about upcoming new TV shows 2020.