The British actress was working on the comedy drama when director Lynn Shelton asked her co-star Rosemarie DeWitt to make her cry tears of laughter for a scene.
"I remember one scene when the director Lynn Shelton came up to Rosemarie and whispered in her ear, 'Say something that would really embarrass Emily'," she told Elle.
"So Rose told this story that involves my nether regions... Basically, her friend was going out with this boy and he'd said to her, 'I think you need to shave your pubic hair because it's really pouffey in your underwear', and so we're doing this scene and Rosemarie just goes, 'Do you remember when Chris told you to shave your bush?'"
She added: "I went completely crimson, I was so mortified. For the first time ever on film, I cried with laughter and they caught it, they captured that golden moment when it was completely genuine."
Blunt also spoke about how she laughed hysterically when meeting US President Barack Obama for the first time.
"I was so starstruck... we were at the White House having cocktails and this woman said, 'Would you like to meet President Obama?'
"They introduced me and then he turned to me and went, 'Oh... Oh... You are a wonderful actress'. And I laughed, screechingly and hysterically, I couldn't stop.
"They had to usher me away. I fell apart. I don't know what happened to me. He looked nine-feet tall. I think my eyes rolled back in my head. I gave Michelle a hug and she had skin like silk."
Emily Blunt recently said that her marriage to Office actor John Krasinski is her main priority in life.
> Emily Blunt: "Maybe I need to be James Bond"
Watch Emily Blunt in the trailer for Salmon Fishing in the Yemen below: