The 42-year-old actress, who was recently rumoured to be involved in a possible Beetlejuice sequel, looked elegant in a flowing cream dress and simple black blazer as she attended the event hosted by Vogue at the Cinema Society.
The star posed with and embraced Nighy and the show's writer Sir David Hare on the red carpet.
Ryder plays Melanie Fall, a businesswoman with a secret, in the BBC original drama from Oscar-nominated writer Hare.
Meanwhile, Nighy reprises his role of ex-MI5 operative Johnny Worricker in the follow-up to 2011 thriller Page Eight.
Christopher Walken also stars in the spy thriller as CIA agent Curtis Pelissier, while Helena Bonham Carter plays Margot Tyrell, an old girlfriend of Worricker's.
Dylan Baker and Sherlock actor Rupert Graves appear as well in the 95-minute drama,