"I remember it was going to be out sometime in the winter over here [in the US] - like a January release. That's usually when studios just send out the movies they don't care about."
But then the audience test screenings blew everyone away.
"The results of that were so high, it scored better than anything they'd done for so long, and they were like, 'wait a minute - we have something special here'," the actress revealed, adding that the score was "something like 97 out of 100".
"So then it got changed to a summer release ... and the rest is history."
Hannigan, who is coming to Australia next month for Oz Comic-Con, remembers receiving an excited phone call from one of the producers.
"He rang me the next day and went 'oh man, this is going to be big'."
Despite early reservations from the studio, Hannigan, who made a name for herself in Buffy The Vampire Slayer and How I Met Your Mother, always knew they had a hit on their hands.
"I loved the script - I loved Michelle as soon as I read the part. I just adored her. Then I met the rest of the cast, and we did a table read that was even better than the script," she said.
"It was like that at every step - it just got better and better. I was just so excited for my friends to see it, because I knew they'd love it."
Hannigan also confessed she was the brains behind one of her character's raunchiest lines.
To refresh your memory: when Michelle and Jim (Jason Biggs) are in bed together at the end of the movie, she famously cries out "Say my name, bitch!" before slapping him.
"It was improvised - and we were so excited because everybody burst out laughing as soon as they called cut, I was really hoping they'd use it," Hannigan explained.
But there were concerns about keeping the line in the movie.
"They [producers] were saying 'Oh, I don't know, I don't know if we'll be able to use that' ... I was like, 'Um, have you seen the movie? Come on!'" she laughed.
"So I was really glad when it stayed in."
But of course, it's the lines about band camp, her flute and a certain sexual activity for which Hannigan is now most famous.
... And 20 years on, it still haunts her.
"The other day I was having breakfast with my kids in a restaurant, and this guy leans right over into our booth," Hannigan recalled.
"He was trying to get his son to recognise me, and was being really obnoxious about it, he's like 'It's the girl from band camp!'"
Of course, when he finally left, her young daughter immediately asked about it.
"I was hoping to avoid that for a few more years," Hannigan admitted.
