The pair became close during awards season in 2010, when they were both nominated for multiple gongs, Colin for his role in 'A Single Man' and Carey for her breakthrough performance in 'An Education'.
A source told The Sun newspaper: "She thought it would be funny to have the actor address the room.
"Unlike his character in 'The King's Speech', Colin was a pro at the speech. He was his charming self and had the room in fits."
Carey - who wore a full-length ivory plain backless dress with V-straps, thought to be a Versace design - had eight bridesmaids, wearing turquoise knee-high prom dresses.
Among the 200 guests at the rustic-themed celebration were celebrities including Sienna Miller and Carey's 'Brothers' co-star Jake Gyllenhaal.
Carey - who previously dated 'Transformers' actor Shia LaBeouf - and Marcus were childhood pen pals who reconnected as adults.
They have been dating for over a year and got engaged last August, though they are rarely seen in public together.