The Love Actually star is engaged to wed Hannah Dodkin, who he began dating at the start of 2010.
But Marshall admits he wants a low-key ceremony after playing a groom earlier this year (11) in TV commercials for U.K. telecommunications firm BT.
He says, "Just filming the scene with the white tables and speeches made me feel ill. Even pretending to get married like that was awful.
"Fortunately my gorgeous fiancee feels exactly the same so we're going to do things very differently. We're not quite sure what but it won't be big."