The actor, 23, who plays Finn in the hit E4 drama, said the Scottish actress put a huge piece of fur down her knickers in the scene where Finn fingers Rae.
He said: “Sharon did this thing where she put a bit of fur under her pants, so when I put my hand there it was like a baby lion. That broke up the tension a little bit."
Mirallegro added that the sex scenes weren’t embarrassing to film.
“They don’t bother me, as long as both of you know what you’re doing and you’re comfortable,” he said.
Rooney, 25, who stars in the series as Rae Earl, said she enjoyed abusing her friendship with Mirallegro by playing tricks on him.
“Working with Nico, there’s nothing to be embarrassed about. He’s a great friend, he’s so kind, so I abuse that by putting funny things everywhere!”
The two star as on-screen couple in the drama, who recently broke up because Rae thought Finn was too good for her.
Speaking at the Royal Television Society Awards last night, Mirallegro said he wanted the show’s audience to understand that relationships are not based on looks.
“I don’t think Finn is too good looking for Rae at all,” he told The Independent.
“I want the audience to think that Finn understands Rae and that’s the type of girl he’s looking for and they both get on with each other in the same way and both love each other’s company, and they love music and all that kind of thing.”
My Mad Fat Diary continues on E4 at 10pm on Monday.