Immediately after watching her turn as a foulmouthed bridezilla in a rough cut of “Dirty Grandpa,” Julianne Hough felt she had some explaining to do to her future off-screen husband.  “I was like, ‘Hey, Hon, when you see this movie, don’t get any ideas. This is not who I am,’” Hough told the Daily News about the conversation with her off-screen fiancé, Washington Capitals forward Brooks Laich.  The R-rated comedy about Hough’s henpecked groom-to-be (Zac Efron) — coerced into a road trip to Florida, complete with pit stops for debauchery by his recently widowed grandfather (Robert De Niro) — opens Friday.  “‘Dirty Grandpa’ is probably the dirtiest movie I’ve ever seen,” said the 27-year-old dancer-turned-actress. “It’s hilarious and revolting at the same time. ... My grandparents on both sides of the family are Mormons, so I’m pretty sure they’re not going to see it.”  But, she added, it’s nice to quickstep away from her girl-next-door image in previous movies like “Footloose” and “Safe Haven.”  “I might not talk like that all the time,” she said of the new role, “but I can let some F-bombs slip from time to time.”  There’s no reason for the “Dancing With the Stars” judge to curse nowadays. She’s currently rehearsing hard for the Jan. 31 telecast of “Grease: Live” — playing the part of Sandy made famous by Olivia Newton-John in the 1978 movie musical.  Growing up with a religious family in Utah, she wasn’t supposed to see the original “Grease” — but it still changed her life when she first watched it clandestinely at the age of 6.  “My parents had like a stash of PG-13 movies upstairs in their closet that we weren’t allowed to watch,” she said, “and whenever they’d be out of town or at work late, I would watch it over and over and over again.  “I’d just learn all of Sandy’s moves and the songs. It was the first time I actually thought I could dance and sing and act and do this as a career —How (that) I could do this in real life and not just pretend in my basement.”  The career choice has worked out pretty well for Hough. From an early age, she excelled on the professional ballroom dancing circuit. She landed her first brush with the film business as a young extra in 2001’s “Harry Potter and the Sorcerer’s Stone.” (That’s her cheering for Gryffindor next to Ron in the first Quidditch match).  She took her first real steps toward stardom, however, years later with the reality dancing competition series “Dancing With the Stars,” where she currently serves as a judge. On that show, it all came full circle recently.  “Olivia Newton-John actually was a guest judge on ‘Dancing With the Stars’ maybe a month ago and she was just like, ‘I hear you’re doing ‘Grease.’ You’re a perfect Sandy. You’re going to be amazing.’  “I thought, ‘Oh my gosh, I think I just got her blessing!’ ”
Julienne Hough stars in "Dirty Grandpa," a movie she may not want to show off to her family.