The fictional series is themed around the cutthroat world of the television industry in the 1980s.
Robbins spoke to BBC Three Counties Radio about her experiences judging their upcoming Make a Difference Awards, when she revealed she had joined the show.
“Wait until you see the new series, I can’t talk about anything… It’s amazing,” she said.
The first season, aired in October, won two BAFTA TV Craft Awards.
Atack plays Sarah Stratton, who aspires to be a television presenter.
Robbins explained that Atack had just had a baby, who she had to bring along to filming, but that she babysat for her daughter “for a couple of days here and there”.