The film's director Richard Curtis has reassembled his all-star cast for a Comic Relief TV 'sequel', with Liam Neeson, Hugh Grant, and Bill Nighy all on board - but Thompson, who played Karen in the 2003 film, won't be part of the get together, reported Daily Mirror.
"Richard wrote to me and said, 'Darling, we can't write anything for you because of Alan', and I said, 'No, of course, it would be sad, too sad'," Thompson said.
"We thought and thought (about it) but it just seemed wrong to revisit the wonderful fun characters of Bill Nighy and Hugh Grant and Liam (Neeson) and all of that... It was absolutely the right decision."
Rickman died in January, 2016, aged 69.
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Asked what she thinks her character would be doing now, Thompson said, "I would be working on some kind of ward."
Curtis' 10-minute sequel, titled Red Nose Day Actually, will air in Britain during the BBC's Red Nose Day broadcast on March 24. The short film will revisit the characters from the original 14 years later.