GQ fez uma entrevista com McCarthy, Ben Falcone (que interpretou o marechal e marido de McCarthy), o diretor Paul Feig e grande parte do resto do elenco. Vale a pena ler (e dar uma olhada na foto incrível que vem junto com ele), mas aqui estão os bits que são relevantes para a sua pergunta:
Paul Feig:
During the Brazilian-restaurant scene, Helen's talking about this big house she has and that people can stay with her if they want. Melissa goes on this whole run of how she needs to do that because there's a squirrel infestation at her house, and a squirrel had burrowed into her vagina and was living in her vagina. I could never get it into the movie, but it made me laugh so hard.
Feig novamente:
The scene [in which Megan, stricken with diarrhea, screams,] "Look away!" [from atop the bathroom sink] was in the script, but there was this whole thing she improvised where she's suddenly screaming at the dress-shop owner, "Give me your fucking jacket!" She wanted to wipe her ass with the woman's jacket.
Ben Falcone:
From the skeleton of the script, the plane ride became wildly improvised. Melissa's talking about putting a Nano up my butt, or up her butt—the Nano was going up someone's butt. I ruined millions of takes by laughing. In the scene outside the bathroom, she had to say, "Let's go to the restroom—and not rest," and after that she was free to improv. She had such good ones that I ruined. Like she said, "Do you like this leg? I got another one just like it. I can put them both over my head and comb my goddamn hair."
Por isso, parece que McCarthy recebeu um pouco de liberdade para improvisar - o que, na minha opinião, foi uma decisão extraordinariamente boa.
(Há também algumas evidências de improvisação no rolo de papel do filme, de McCarthy e de boa parte do restante do elenco.)