Prime Video's Young Sherlock has added four new series regulars for its second season, and the names carry real weight. Olivia Williams, Sophie Skelton, Indira Varma and Aidan Gillen have all joined the cast, according to Deadline Deadline. Character details for all four are being kept under wraps for now.

The show, created for television by showrunner Matthew Parkhill and co-developed with Guy Ritchie, proved a sizable hit in its first outing. Season 1 reached 45 million viewers in its first 28 days, hit number 1 in over 95 countries and ranks among the top 10 Prime Original seasons of all time. Sixty-three per cent of that audience came from international markets, with the UK, India and Germany over-indexing.

Season 2 is already in production. Cameras started rolling in late May, with Palermo, Italy serving as one of the filming locations. Ritchie, an executive producer on the series, is directing the first episode of the new season.

Several familiar faces are returning. Hero Fiennes Tiffin leads as the young Sherlock Holmes, with Dónal Finn back as James Moriarty. Max Irons returns as Mycroft Holmes, Sherlock's older brother. Natascha McElhone is back as Cordelia, Sherlock's mother, and Holly Cattle returns as Beatrice, Sherlock's sister.

Not everyone is coming back. Zine Tseng, who played Xiao Wei (also known as Princess Gulun Shou'an), will not return. Joseph Fiennes, who played Sherlock's father Silas Holmes, is also out. In the Season 1 finale, Silas jumped off a cliff and was presumed dead, though that was never confirmed on screen. Numan Acar, who played the fixer Esad Kasgarli, is another departure. Colin Firth's character, Bucephalus Hodge, was killed off earlier in Season 1.

For Skelton, the role marks her first television work since wrapping a seven-year run as Brianna Randall Fraser on Starz's Outlander. Williams is known for films including The Sixth Sense and Rushmore, and recently played Nurse Ratched in a stage adaptation of One Flew Over the Cuckoo's Nest at London's Old Vic. Varma appears as Mrs. Gardiner in The Other Bennet Sister and recently starred in Season 2 of The Night Manager for Prime Video. Gillen's credits include Game of Thrones and Peaky Blinders.

For fans, the new additions signal a fresh chapter for a show that clearly found its audience. With four seasoned performers coming in and several cast members moving on, the shape of Season 2 is starting to come into focus, even if the character details are not.