Black Mirror actor Tom takes on the role of Lord Anthony Gillingham, who is a potential suitor for Lady Mary, while former EastEnders actor Nigel stars as Gillingham's valet, Green.
"My name is Green but I get called Gillingham by the other staff," explains Nigel. "Because when you're downstairs you get called the name of your principal."
Tom Cullen (left) plays Lord Anthony Gillingham and Nigel Harman (right) stars as his valet, Green
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"He's very much a social animal and he loves the people that are around him," added the 40-year-old. "He loves having a good time and a laugh and quite often palms off his work to other people so he can spend time hanging around them."
His character will be at the centre of a dark, new story that also involves Anna, who is Lady Mary's maid, and her husband Bates.
Nigel, who has enjoyed theatrical roles in Shrek The Musical and Guys and Dolls, also spoke of how he felt joining the multi-award-winning show.
"It's been brilliant and slightly alarming," he said. "There's a responsibilty with a show that's this big, particularly when you've got a strong storyline that you want to reach the marks."