The British actress, whose big screen credits include 28 Days Later and Pirates of the Caribbean, leapt into the James Bond franchise as MI6 field agent Eve, a role shrouded in secrecy by the film's production team.
After multiple auditions, Harris was told by director Sam Mendes and producer Barbara Broccoli that Skyfall would reintroduce several key characters from the series' past, and she would step into a role previously played by Lois Maxwell, Caroline Bliss and Samantha Bond.
"I had to keep everything a secret for ages," Harris told Digital Spy during the inaugural East Coast Trains journey from London to Edinburgh.
"Even when I'd just got given the role I was told that I couldn't tell anyone, so it's nice now to be free of all [the secrecy] and just say, 'Yes, I am Miss Moneypenny!'"
Harris admitted that her favourite scene to film was the cutthroat shaving sequence with Daniel Craig because it provided an "intimate" break from all the high-octane action set pieces.
The moment has prompted much debate among 007 fans about whether or not Bond and Moneypenny spent the night together.
Harris offered up her own theory on the ambiguous scene, saying: "In the script they don't because they get interrupted and it never happens.
"That was cut out of the script and it was left up to the viewers' imaginations, but in my mind nothing happened. I think it would spoil their relationship, it's nice that they have that sexual tension that they've never actually fulfilled."