The singing sensation, 20, picked British-born actor Toby Hemingway, 27, to walk her down the aisle in the video for her soon-to-be-released song, "Mine," in which she plays a bride.
"I saw Feast of Love and just loved that movie and thought Toby was so great that I thought it would be perfect to put him in the video," Swift tells PEOPLE about the Los Angeles-based actor.
But it was the unexpected appearance of her lucky number that sealed the deal.
"I was doing some more research and watched another movie he was in called The Covenant,” she says. "I've got this crazy 13 lucky thing, and he walks on screen for the first time wearing a sweatshirt with a 13 on it! That was the deciding factor. It wasn't really up to me, it was about the number."
The costars seem to have clicked in person, too – they were recently seen hanging out together, when Swift was in L.A. to record with an orchestra for her next album, Speak Now, due Oct. 25.
But getting married on film "was a little strange," Swift says. "I get to wear big dresses and wear white all the time on the carpet, but there's nothing quite like actually pretending you're getting married."
If the message in her song "Mine" – out Aug. 16 – is any indication, it’ll probably be a long time before she's ready for the real thing. "It's a song that is about my tendency to run from love – it's been a recent tendency," she told viewers during a live Web chat Tuesday.
"Every really direct example of love that I've had in front of me has ended in goodbye, has ended in breakups," she said. "So I've developed a pattern of sort of running away when it comes time to fall in love. This song is sort of about finding the exception to that."