The "Knocked Up" star partnered with the British Oscar winner in the new film version of Noel Coward's play "Blithe Spirit", and really lost the plot when she got up close and personal with the great Dame.
"I was terrified," Leslie told America's "Live with Kelly and Ryan". "It's Judi Dench and she's, like, one of my heroes. The first time we worked together, I didn't get a chance to really talk to her much but they just kind of put us on a bed together because I'm a ghost haunting her while she's trying to sleep..."
"I was sweating and nervous and I couldn't remember my lines. It took me about two hours to calm down and be able to film the scene with her."
Even worse, she had no idea how to address the Dame.
"I didn't know what to call her," she added. "It didn't feel right calling her Dame Judi or Mrs. Dench. I didn't know how to refer to her."
But Judi Dench helped Leslie Mann calm her nerves. "I think she knew I was nervous, I was probably sweating or something, and she went out of her way to make me feel comfortable and she set me at ease," she said a previous interview.
Aside from struggling with her jittery when sharing scene with the Dame, the actress enjoyed most of the filming. She especially relished wearing the period outfits. "It was exciting to learn something new and a new way of doing things. All the costumes were so much fun, incredible to wear something other than what my characters would normally wear like jeans or t-shirts," she said.