Former E.R. star Linda Cardellini was banned from attending the Mad Men season premiere party so reporters would not know she was joining the cast as ad man Don Draper's neighbourly lover.

Series creator Matt Weiner swore the new mum to secrecy about her role on the show and even forbade her from discussing her involvement with family and friends.

Jon Hamm, who plays Draper on the show, says, "She kept it incredibly sealed. We had the premiere and she was not at the premiere because if she showed up, people would be like, 'What are you doing at the premiere?'"

And her onscreen lover insists the bid to keep Cardellini's casting a secret worked out perfectly: "As soon as the cat was out the bag it was amazing, people were freaked out and duly surprised."