The mysterious relationship between actors who play on-screen lovers has long been conversational catnip. For audiences, the believability of attraction is a topic that approaches almost academic analysis. Such chemistry tends to be viewed as an absolute. By common agreement, a couple either have it (Humphrey Bogart and Lauren Bacall in, say, To Have and Have Not) or they don’t (Brad Pitt and Angelina Jolie in By the Sea).

La La Land, a frontrunner for next year’s best picture Oscar, reunites Ryan Gosling and Emma Stone as lovers for the third time, following Crazy Stupid Love and Gangster Squad. Here, they sling fiery one-liners at each other, break into song, and dance along a Hollywood avenue. They, it seems, have it.

The whole film nods to the past; the Golden Age ancestors of their pairing are enthusiastically invoked both by the movie itself and its publicity. Director Damien Chazelle has said: “They feel like the closest thing that we have right now to an old Hollywood couple,” comparing them to Spencer Tracy and Katharine Hepburn, Ginger Rogers and Fred Astaire.

But what are the elements that enhance or undermine the apparent spark of two actors Velcro-ed together because of a casting director, an offer too lucrative or career-enhancing to resist, or even a well-written script? And can these celluloid relationships really be viewed objectively? It’s … complicated.

As a young actor, I experienced professional kissy-face moments that were supposed to be hot, and were surprisingly convincing on-screen despite never getting past lukewarm. There were others that were supposed to be more contained than natural, hormone-driven lust would allow, and disappointed in the finished product; and (most regrettably) some that – whether or not they worked on screen – made me feel as if I had eaten a dirt sandwich.

As the spouse of an actor, Billy Connolly, I confess I have slammed the odd door on discovering that my husband’s location film work included a bedroom scene with (to name just one) Sharon Stone. As a director, I have noticed the insidious way in which my own alchemical manipulation of two porous beings can influence the quality of their on-screen coupling, and as an audience member (and, OK, psychologist) I am well aware of the way my own projected fantasies can create the illusion of passion or puke.

Actors are trained to open themselves up to instant intimacy with strangers. Depending on their type of training, they often take highly personal aspects of themselves, such as their individual courting style, their own erotic response, and perhaps even their physical nakedness, bestow those qualities on the character they are playing, and try to engage in a heightened connection with a stranger who is attempting the same process in relation to them.

Forbidden and hot love: Keanu Reeves and Aitana Sánchez-Gijón in A Walk in the Clouds, 1995. Photograph: Moviestore/Rex/Shutterstock

It is never easy, but such psyche-bending can be a lot easier if there is a natural attraction. For this reason, during casting readings, directors and casting directors will look for physical signs of natural attraction between actors, such as enlarged pupils, open body language, giggling, skin flushes, or heightened awareness of each other. If some of these basic signs are present, one may only have to add the proximity of weeks or months on set together, and the willingness to have a creative success, to cause a spark that transcends the workplace.

Since one of the most compelling elements of erotic attraction is the taboo of illicitness (any work situation qualifies), it is little wonder that it is hard to put the genie back in the bottle when the shoot is over. It is even more difficult when truly intimate scenes are required in the filming. Some actors lack the training, personal boundaries, or psychological health to avoid being lost in the mire of a celluloid love bubble. Frankly, Hollywood insiders regard it as a cliche – creating the kind of real/artificial relationship that will make an audience suspend their belief that the two people moaning in the shower are just jobbing actors, can be psychically, psychologically, even sexually dangerous – and frequently damages real-life liaisons.

In fact, there are many different workplace hazards for romantic leads. The ordeal of courting or making love in front of just a piece of machinery – a camera – might be a lot easier, but these days even a closed set or “skeleton crew” probably involves 15 people, whose attitude to the scene might range from overreaching voyeurism to wondering when lunch will be served.

So which of cinema’s couples are considered the most sizzling? Common answers to that have often been tainted by the machinery of hype and promotion, and do not necessarily tally with audiences’ real feelings. Take Cleopatra. If you had seen that movie just after waking from a 20-year coma, lacking knowledge of the massive publicity surrounding the personal illicit romance between lead actors Elizabeth Taylor and Richard Burton, you surely would have been unconvinced by their on-screen connection.

Dancing partnership but no kisses: Fred Astaire and Ginger Rogers in Top Hat. Photograph: Moviestore Collection/Rex Features

Astaire and Rogers? OK, that was a marvellous dancing partnership, but what about romantic interest? At their exceptional level of dance, personal affection between partners is often forfeited amid the pressure of perfecting steps, dealing with pain and injury, and surmounting creative differences. According to post-partnership interviews, Fred and Ginger were no different. And Mrs Astaire reportedly made sure a “no kissing” clause was always in Fred’s contract.

What about Patrick Swayze and Jennifer Grey in Dirty Dancing? There is the same sense of, “it’s more about the dancing” and “young, rebellious triumph over authority and adversity” than a believable love connection.

Of course, these are simply subjective impressions. But I would argue that on-screen chemistry can never be regarded as objective or absolute: the real nature of it is always very much in the eye of the beholder. Yes, the audience can generally read subtle physical signs of genuine attraction between two actors, and appreciate the truth of their whispered admiration, but whether or not it really grabs them depends on their own attraction and relationship “maps”. Likewise, whether or not one identifies closely with one of the actors is also an important factor (demographics such as race, age and education play a part), as are one’s feelings about the pair’s situation, one’s judgment about whether or not they are well-matched, one’s perception of their physical beauty, and so on.

More dancing than love: Patrick Swayze and Jennifer Grey in the 1987 film Dirty Dancing. Photograph: Allstar/Vestron Pictures

Whether or not one is personally swept up in the story and the power of the dialogue will have an enormous effect, too. It is not really about them; it’s about us. Which couple grabbed you more, and why: Meg Ryan and Tom Hanks in Sleepless in Seattle (“It was a million tiny little things that, when you added them all up, they meant we were supposed to be together”); Tom Cruise and Kelly McGillis in Top Gun (“I don’t date students”); Heath Ledger and Jake Gyllenhaal in Brokeback Mountain (“I wish I knew how to quit you”)? Your demographic is one of the key predictors of whom you will enjoy most. Not Robert Pattinson and Kristen Stewart in Twilight? How about Will Smith and Eva Mendes in Hitch (“Life is not the amount of breaths you take, it’s the moments that take your breath away”)? Target audiences vary from movie to movie, and some couples are considered more successful simply because they are more likely to engage a wider demographic than others.

An especially compelling kind of on-screen relationship is one in which physical attraction is masked by the tension of mock-hatred, and expressed through witty banter. Witnessed perfectly in the coupling of Hepburn and Tracy, in Adam’s Rib and eight other classic movies in which they were paired, this style has been recreated in La La Land. Barbed dialogue is a means of being inviting and rejecting at the same time, of maintaining decorum appropriate to the movies of the 40s, of setting boundaries against overt sexuality via tone. It is strange to see this executed by young, contemporary actors such as Gosling and Stone; yet they do it so well that they almost convince you it is new and cool. Personally, I find this style of connection somewhat alienating, except in the case of Harrison Ford and Anne Heche in Six Days, Seven Nights, where the protagonists managed to balance witty one-liners with down-and-dirty, despite-themselves lust. (“Aren’t you one of … those guys? ... You send them into the wilderness with a pocket knife and a Q-tip and they build you a shopping mall.”)

Man to man: Jake Gyllenhaal and Heath Ledger in Brokeback Mountain. Photograph: Focus/Everett/Rex Features

I like my movie romances raw. So, Hugh Grant and Martine McCutcheon in Love, Actually – low buzzer. Kate Winslet and Leonardo DiCaprio in Titanic? Also low buzzer (they seem more like siblings to me). George Clooney and Michelle Pfeiffer in One Fine Day? Well-matched, mature lovers – score! Keanu Reeves and Aitana Sánchez-Gijón in A Walk in the Clouds? Forbidden, hot. Ditto Russell Crowe and Meg Ryan in Proof of Life. But that’s just me. The truth is, each audience member projects their own, particular, deep-seated longing, lust, and love-aspirations on to whichever couple displays the right character, appearance, behaviour, situation, and relating style for their individual tastes. They are part of a three-way relationship: in a triangle with the two actors.

Hollywood knows what it is doing. It is a science now. There is far less guesswork than there used to be. The endless focus groups tell the big studios which actor couplings appeal to which demographic; they are looking at who is likely to play best with the majority of people in target audiences. What they capture during the shoot is not overly important – they can tweak the film in post-production. Thirty different cuts can be graded by discerning previewers. A low score on a particular line heralds a fast reshoot, while marks for two or three different endings point the way to the final cut. So let’s not be naive. Creating chemistry is a massive job shared by hundreds of smart people, some of them utterly cynical about how to compel an audience with a thrilling, romantic tale told directly to your pocket.

But we need entertainment and escape. So don’t let me ruin it for you. Go ahead. Suspend belief. Get lost in a magical movie romance and don’t fret that it’s really a mini Rorschach-type test for your own longing and desire. Although, a touch of cynicism is a healthy thing. As Nicolas Cage says to Cher in Moonstruck: “We are here to ruin ourselves and to break our hearts and love the wrong people and die. The storybooks are bullshit. Now I want you to come upstairs and get in my bed.”

• La La Land is released in the UK on 13 January 2017.