Actors are able to fake emotions that many of us can barely control.
They can create a facsimile of love, hate, happiness or sadness out of thin air.
In the case of Fifty Shades Of Grey, the challenge was perhaps even harder.
Stars Dakota Johnson and Jamie Dornan had to whip up earth-shattering erotic chemistry, without which the whole project would be an utter failure.
The degree of difficulty was increased exponentially by the fact that Johnson, 25, and Dornan, 32, appear to have no real-life heat whatsoever.
During their recent press tour, in joint interviews and on red carpets, the awkward duo have come across more as siblings than lovers.
Body language-wise, there is always a space between them. They are wary of touching each other even in a friendly way, even when co-presenting at last month's Golden Globe Awards.
Dornan is quiet and almost shy while Johnson is cheeky.
He is from Northern Ireland, his father an obstetrician and his grandparents, Methodist lay preachers.
She is a Hollywood kid, a third-generation actress, born to Miami Vice TV star Don Johnson and movie actress Melanie Griffith, and even served as Miss Golden Globe back in 2006.
When asked about their sex scenes during their very first joint interview on the Today show back in July 2014, Johnson said: "It's not a romantic situation. It's more technical and choreographed... It's more of a task."
Dornan, sitting stiffly at her side, just laughed.
The message they are sending could not be more clear - they are not Robert Pattinson and Kristen Stewart.
There are no behind-the-scenes shenanigans here. In a word, their relationship appears strictly professional, which is really as it should be, especially when you are making a movie with so much kinky hanky-panky.
Based on the novel trilogy by E. L. James, Fifty Shades Of Grey - which is showing in cinemas - tells the story of Anastasia Steele (Johnson), a young woman who is swept off her feet by handsome billionaire Christian Grey (Dornan), who is into bondage and wants to make her his "submissive".
It is easy to see how a couple of actors could be cagey and unnaturally cool with one another after playing characters in such a bizarre and intimate situation.
In fact, some have speculated that Dornan and Johnson secretly hate each other.
Especially damning is a recent video interview with Glamour magazine in which they are asked to describe each other in a few words, and struggle to find anything nice to say at all.
Dornan takes several seconds to come up with the bland, generic, impersonal adjectives "funny", "talented" and "caring".
Johnson takes several seconds to come up with "superb", which is how you describe a soup, not a human being.
Rumours of a supposed feud have bubbled up to the point that Dornan had to address them at the movie's London premiere.
He told E! News: "Look, people are going to say that because it's a story and people like to create a little bit of hype leading up to the film.
"It's an obvious story to come up with, you know, if I was into the world of coming up with fake stories I'd maybe start to do that on a blog...
"No, I wouldn't, I would never become that person, but I understand it. It creates a bit of... whatever."
The main issue here is that fans have certain expectations about how Dornan and Johnson should get along that are impossible to live up to.
In the world of pop culture, Fifty Shades Of Grey has exploded like a giant sex bomb.
The hype surrounding the movie and its "playroom" scenes have been, in a word, crazy.
It is not often in a mainstream film that we see a man lock a woman up in handcuffs, bind her with ropes and whip her. It is just so insanely, intensely erotic that it strikes people as weird when it does not spill over into Dornan and Johnson's off-screen relationship.
Thing is, for the pair themselves, it seems to be just another acting job - one that happens to also be a huge opportunity.
Dornan has done some good work on TV shows such as Once Upon A Time and The Fall, but Fifty Shades has made him a movie star.
Johnson was a virtual unknown, having appeared in small roles in flicks such as Beastly, 21 Jump Street and Need For Speed.
Now, she is the talk of the town, nay, the world.
The other important thing to remember is that Dornan is a happily married man. For him, the sex was not a selling point but a bit of a hurdle.
"I think there's some stuff where my family and friends will never have seen that side of me before," he said in an interview with the London Evening Standard, "but it wasn't enough to make me not want to do it."
Dornan's wife, British actress-singer Amelia Warner, has yet to see the movie and Dornan is more than fine with that.
"She hasn't seen the film and I don't think she will, to be honest," he said at the film's UK premiere.
"I'd understand if she didn't want to."
Dornan and Warner got married in 2013. She gave birth to their daughter Dulcie while he was filming Fifty Shades in Vancouver, Canada, last year, a period of his life that he describes to Vogue magazine as "totally crazy".
As for Johnson, she has remained mum on her love life, though she has been linked with musician Matthew Hitt, with whom she has been spotted since last July.
It is obvious that Dornan and Johnson have separate lives.
That said, they are going to be spending a lot more time together over the next few years.
The massive success of Fifty Shades Of Grey - it brought in a staggering US$266 million (S$360 million) at the global box office and over S$900,000 in Singapore over its opening weekend - has ensured that we will see the two sequels, Fifty Shades Darker and Fifty Shades Freed, on the big screen.
Johnson told Vogue that she would like to take the rest of the year off and catch up on her sleep.
But that would seem to be a dim possibility at this point.
After all, there is more work to be done in the playroom.