One of the pitfalls of making an adult-rated comedy for an actor is having to explain a graphic sex term to your mother when she visits the set.
That was the awkward position Ed Helms found himself in while filming Vacation, about the Griswold family's disaster-filled road trip to an amusement park.
A sequel to the beloved National Lampoon's Vacation movies starring Chevy Chase (1983 - 2010), the film - which opens tomorrow in Singapore with an M18 rating - features a running joke in which a teenager misunderstands the colloquial term for a certain sexual act.
Helms, 41, best known as one of the stars of The Hangover trilogy (2008 - 2013), says: "My mother, who was visiting the set that day, did not know what it was. So I had to explain. It was terrifying, but she took it well." He plays Rusty Griswold, a character who was a boy in the earlier films, but is now a grown man dragging his own family on a nightmare holiday.
For the film-makers, a drawback to Vacation's raunchy humour may be that they have to answer questions about the sizeable prosthetic penis worn by cast member Chris Hemsworth, who plays Rusty's handsome brother- in-law.
First-time directors John Francis Daley and Jonathan Goldstein, who also co-wrote the movie, describe the "surreal" process of selecting an appropriately sized prop followed by the embarrassment of shooting scenes with it for the 31-year-old Australian star as well as for his stand-in and the crew.
At the press event in Los Angeles to promote the film, a Belgian reporter oblivious to the fact that it was clearly a prosthetic, remarked that "the penis is pushing the envelope really far".
Goldstein gently tells him that it is fake, but seeing his disappointment, adds: "But what does Belgium need to hear? You can write that it was real if you want."
Daley says helpfully: "Whatever creates the most buzz."
The directors may have a bigger problem on their hands, however: Their film has had a limp debut in the United States, where it has earned just over US$21 million (S$28.9 million) since opening last Wednesday - far less than many were hoping for, given the US$38 million Jennifer Aniston's We're The Millers, a family road-trip comedy with similar humour, made over the same period last year.
This makes Vacation the latest in a series of R-rated comedies to underperform in the country, along with Entourage and Magic Mike XXL.
Daley and Goldstein could have made a movie with a lower rating and captured a larger audience that way. But they say they wanted to keep some of the edgier gags to avoid falling into the trap of making "soft comedies where you're not really creating any reaction", Daley says.
In the US, under-17s have to be accompanied by a parent or adult guardian to watch R-rated movies, while PG-13 warns parents some material may not be suitable for under 13s.
The risk of certain scenes in Vacation offending some members of the audience was one that the filmmakers and Helms were prepared to take, they say. Goldstein says one particularly nauseating scene - where the Griswolds mistakenly take a dip in a pool of sewage - divided test screening audiences down the middle.
"Half the audience were so disgusted, they could barely watch it, the other half thought it was the funniest thing in the world. It's a very polarising scene, which any edgy comedy is going to do - it's not for everybody."
With major Hollywood studios making fewer comedies than they used to, and the comedies they do release earning less at the box office, it is even harder to make a film like this a success.
But keeping the budget of Vacation relatively low at US$31 million "takes some of the pressure off that opening weekend", Daley says.
Goldstein adds: "I think it forces you, if you're going to make a comedy, to keep the budget in check. This was not a super expensive movie by Hollywood standards and we wanted to keep it that way."
The film-makers are proud of another accomplishment - their discovery of a new comedy star in Hemsworth.
Daley says: "The studio pitched him to us - his people said he wanted to try his hand at comedy because he was interested in straying from the action movies he's been doing.
"We had no idea what to expect when he first came on board, we had never seen him do anything like this before. And when we started working with him, it was such a pleasant surprise that he completely got the tone of the movie and never tried too hard to be funny, it came naturally."
Helms is also full of praise for Hemsworth, but admits being miffed that the handsome Australian star turned out to be so good.
He says: "He's really funny, which is really annoying for me because it's just not fair. I spent my whole career trying to develop my comedy chops and Hemsworth comes in with no comedy background and just crushes it. And he has an amazing six-pack."
- Vacation (M18) opens in Singapore tomorrow.