In a profile for GQ, the Mike & Molly actress revealed that she loved Megan's unconventional personality as well as her terrible wardrobe.
McCarthy said: "I read the script and had such a weird, fast reaction to Megan. I knew exactly what I wanted her to look like, to sound like, and I got an intense feeling of 'I like her so much! This will never work out!'
"For the audition, I wore no makeup and a bad pant. Once I look like the character, it really locks me in, even if people sometimes say, 'We need you not to be repulsive'."
McCarthy also spoke about Megan's contradicting presentation of her sexuality, explaining: "I love those no-bulls**t women with close-cropped hair that you'll see together and think, 'Is that her partner?' Then they talk about their husbands and six kids.
"I just love anybody who's that comfortable in her own skin. But the whole ride home from the audition, I was thinking, 'I got too weird'. Should I turn the car around and do that cheesy actor thing of 'I can do it better! Give me another shot!'"
McCarthy recently signed up to star in Judd Apatow's Knocked Up sequel alongside Paul Rudd and Leslie Mann.
She has also been nominated for a 2011 Primetime Emmy Award for her work in Mike & Molly.
Watch a trailer for Bridesmaids below: