The 'Office' star - who married the British actress in an intimate ceremony at Italy's Lake Como in July 2010 - instantly knew he was going to "fall" for the 'Devil Wears Prada' star and told her the first time he ever spoke to her.
He said: "It was one of those things where I wasn't really looking for a relationship and I was thinking I'm going to take my time in Los Angeles.
"Then I met her and I was so nervous. I was like, 'Oh god, I think I'm going to fall in love with her.' As I shook her hand I went, 'I like you.'
"It's one of those things where as soon as you meet someone you kind of know."
When the 31-year-old actor finally decided he was going to propose to Emily, 28, he came up with the idea to do it before she realised she could find a better man.
During an appearance on 'The Ellen Degeneres Show', he joked: "I was like, 'I'm going to propose to her real fast before she wakes and is like, 'What am I doing? I can have anybody.' I was like, 'Just put on this ring. Shhh.' "
Emily recently confessed she is keen to have children with John at some point in the near future and thinks their family backgrounds will make them good parents.
She said: "John's well practiced. I grew up with a younger brother and sister who are quite a bit younger than me. So, at eight years old, I was sitting there with a baby, probably really p***ed off [because] he was puking on me."