The 20-year-old actor finally hooks up with Watson's Hermione Granger in Harry Potter and the Half-Blood Prince, which is due out next month.
"We've known each other for 10 years so it was a bit like kissing my sister," he told the Mail on Sunday.
"We were determined to get it right the first time, but our first try was a disaster because we both felt so self-conscious and we couldn't stop laughing."
While Grint's film love life may be hotting up, the actor says he is in no rush to throw himself into off-screen romances.
Despite the attentions of Harry Potter fans the young millionaire says he is "quite insecure" around girls and has yet to find anyone to suit his "weird" tastes.
"I've never been seriously involved with anyone. I've certainly never been in love. I've always preferred to keep things casual," he said.
"It's not that I find it hard to meet girls – they do come up and chat me up from time to time. Sometimes I'm interested, I tend not to go for the prettier ones. I prefer the quirkier types."
Grint's anxieties are not confined to affairs of the heart. Although lauded by critics for his Harry Potter performances, he admits to doubting his own acting abilities and worrying about his future prospects.
"It's always at the back of my mind that acting might come to an end for me when Harry Potter finishes. I don't know if I'm good enough to have a long career," he said.
"I've got a bit of an inferiority complex about my acting. My self-esteem is quite low in that sense."
