The Glee actor and singer stars in the upcoming ensemble romantic comedy opposite Diaz, with her character being pregnant with their child.
Speaking about his on-screen partner, he said: "She was exactly the way you would expect Cameron Diaz to be.
"She was bubbly and funny and goofy and wonderful, we had a great time."
Cheryl Cole also has a cameo in the film, playing herself as a judge on an X Factor-style talent show.
On the 'Call My Name' singer, he added: "She was great, it was great to meet Cheryl in person.
"I came to London, and you can't escape London without hearing about her. It was great to meet her as a human being. She's so lovely and she's great in it."
Matthew Morrison recently said that he would like to become a father in a "couple of years".
What to Expect When You're Expecting also stars Jennifer Lopez, Elizabeth Banks, Chace Crawford and Dennis Quaid. It is currently playing at US cinemas and opens in the UK on Friday (May 25).
Watch the trailer for What to Expect When You're Expecting below: