But that’s because director Debbie Isitt uses improvisation to spring surprises.
And, having captured some fun with his ears on the day, and the actor himself not objecting, she has left the footage in.
In Nativity 3: Dude Where’s My Donkey?! Clunes plays a super teacher called Mr Shepherd who quickly loses his memory after starting at a new primary school.
Due to get married to Sophie (Catherine Tate), the girl of his dreams who is in New York, he has to find himself first in order to try to win her hand in the Big Apple.
“For Martin’s role I originally thought of someone younger, but because the character loses his memory I went for somebody older,” says Debbie.
“Improvisation needs a brave actor and Martin had loved the first two films with his daughter, who is now 12.
“The fun with his ears came about because it was improvised by Marc Wootton’s Mr Poppy.
“It made me laugh, so I left it in and he didn’t seem to mind it at the premiere.”
Debbie’s first film in the series, Nativity! (2009), cost £2.6 million to make, the second £2.7 million and this week’s third film just £2.8million.
“I’ve been really lucky,” she says of her relationship with film company Entertainment One.
“They have been investing in my films ...but still letting me work in the way I want to.
“So we are making blockbuster movies for less than £3 million – which is unheard of.
“Given that Nativity 2 took more than £10 million at the UK box office alone it shows you how successful the films have become.”
The extra budget for Nativity 3 enabled Debbie to shoot scenes in London and New York.
To give it some extra pre-Christmas oomph at the box office, the release date has come forward a couple of weeks to mid-November.
And so, if all goes to plan, there is talk of a fourth movie going Down Under – because the Aussies love the series as much as anyone.
For Debbie, the franchise remains a labour of love while she tries to fulfil her other ideas for projects including a long-cherished feature set in the seductive world of belly dancing.
“Remember, I got the chance to make Nativity! because of the success of Confetti (2006),” Debbie says of a film which starred Martin Freeman long before he became Bilbo Baggins in The Hobbit.
From first scene to premiere, Nativity 3 took 11 months to make.
Once again, Debbie had already penned the script and the songs with husband and chief collaborator, Nicky Ager.
He has also edited and sound mixed the film by using Apple’s readily available Final Cut Pro software at home.
Influenced by the Technicolor look of her favourite film The Wizard of Oz (1939), Debbie also supervised the colour grading and the animation of the film’s donkey, Archie.
Daughter Sydney has a bigger role in this film as the tall girl of the class, Sadie, and Debbie’s parents Peter, 72, and Barbara, 70, make their customary cameo appearances – though it’s a case of blink and you might miss them this time.
While the new film is Debbie’s favourite because it’s the most recent – she agrees that view might change over time.
“This is the biggest, most ambitious... with the first one I just didn’t expect it to take off.”
A former pupil at Our Lady Fatima RC Primary and Lordswood Girls’ School in Harborne, the now Coventry-based Debbie is proud of how she has made it in a man’s world after writing and directing plays and living on the proverbial breadline.
“I think just three per cent of directors are women which is a very small percentage.
“But there is no reason why more women can’t make films. You just have to believe that you can do it... like a male midwife!”