The duo will appear in upcoming episodes of the show but writer Heidi Thomas remained tight-lipped about their roles.
Speaking at a screening of the first episode today, she joked: "They're both playing women! I'm choosing things very carefully.
"You'll meet Emilia first and Sarah comes in later. I think it's not giving too much away to say that Emilia connects with the upstairs world of 165 [Eaton Place] and [Lancashire's character] connects with the downstairs world of 165."
Thomas continued: "Emilia plays a character called Portia, and Sarah Lancashire plays a character called Violet. Emilia plays a character in her 30s! I really don't want to give the game away because it lets you know things about other characters in the show ahead of time and I think it will spoil it for the audience."
However, BBC Wales' head of drama and Upstairs Downstairs executive producer Faith Penhale provided some hints about Fox and Lancashire's roles.
"I think the other thing it's also worth saying is that they both connect with characters in the house who have sort of space in their hearts for these characters to come into their lives," she teased.
"And I think it's probably also worth saying who they connect to. Portia makes a connection with Blanche [Alex Kingston] and [Violet] makes a connection with Mr Pritchard [Adrian Scarborough]."
Alex Kingston, Kenneth Cranham and Laura Haddock are among the stars who have joined the cast of Upstairs Downstairs for its second series, which will air on BBC One shortly.
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