The British starlet, who is married to James Bond actor Daniel Craig, was happy to talk about the smooch after picking up the EE BAFTA for Best Supporting Actress for her role in The Favourite.
Speaking candidly backstage, Rachel said: “You want to know how I prepared to snog Olivia?
“My mouth met hers and my mouth just opened and I kissed her.
“There isn’t preparation when it comes to kissing someone.”
In the film, Rachel plays the role of Sarah Churchill alongside Colman as Queen Anne and Emma Stone who takes on the role of Abigail Masham.
Gushing over Olivia, Rachel added: “There was no preparation, Olivia is just gorgeous and lovely.
“Snogging Olivia Coleman… I am happy to talk about it.”
During an evening of typical showbiz glitz and glamour at London’s Royal Albert Hall, The Favourite swept the board winning a staggering seven BAFTAs.
As well as winning the gong for Outstanding British Film, Original Screenplay, Costume Design, Production Design and Make Up and Hair, Olivia took home the award for Leading Actress.
The 45-year-old beat out completion from Glenn Close for her role in The Wife and Lady Gaga for her Oscar-nominated film A Star Is Born.
While Gaga decided to snub the ceremony in favour of The Grammys in Los Angeles, Bradley Cooper was on hand to collect the EE BAFTA for Best Song.