Chemistry among actors in movies is arguably about as challenging to teach as learning how to properly execute a romantic mazurka by Chopin on the piano. For those who've ever tried to master the latter, you'd know that it's nearly impossible to teach and requires an intuitive feel of rhythm and structure that an artist either has or not. For the former, it's virtually the same, right down to some actors not knowing how the chemistry with a costar will work until actually shooting a scene together.
Some of the greatest movies about love and romance can give credit to the chemistry of the actors for making the movie a classic. If you didn't think Humphrey Bogart and Ingrid Bergman had any attraction to one another in real life, then perhaps the romantic magic of "Casablanca" may have been diluted for you. Their chemistry has long been a template for how to have two actors appear to be once deeply in love with one another.
Every indication from history shows that those two were seemingly friendly off-camera. Yet they were also two very different people who likely couldn't have had romantic chemistry in real life if they tried. It seems those realms of disparities between actors create an ineffable creative chemistry spark.
You also have another tool that perhaps isn't given enough credit: actors who decidedly dislike one another. Rumors have abounded for decades about various classic romantic pairs in films who absolutely couldn't look one another in the eye or break the ice off-camera without instigating a black eye. Some of those reported rumors include Vivien Leigh and Clark Gable in "Gone With the Wind" and Patrick Swayze and Jennifer Grey in "Dirty Dancing."
Those two examples are arguably some of the greatest romantic duos in movie history next to "Casablanca." Yet if you delve into the realms of psychology, the attraction of opposites is a near constant, leading to some real-life celebrity romances that were too tempestuous to survive beyond a movie set. Movies and TV have also explored two people hating each other so intensely they eventually end up bouncing off the walls in passionate lust.
Conversely, along with romantic movie couples disliking one another in real life, you have actors who have so much chemistry with their costars they end up falling in love in real life. This scenario ends up being even worse in many regards, especially when the real love between the actors is merely brought to life due to the assimilation of the plot or characters. Even though we have some examples of this throughout film history, it's always more fascinating to watch the romance play out onscreen when you know it's not just acting.
But occasionally you'll have romantic chemistry that isn't bred from dislike or truly falling in love. These fortunate actors are usually on the same wavelength when it comes to witty repartee both off- and on-set. Nothing says better chemistry than the interchange of lines and coalescing in each other's response.
This much safer romantic chemistry also makes kissing and/or sex scenes easier to watch, especially when you know the actors didn't share a cigarette afterward or want to cut each other's heads off. It's also a technique that every casting director should watch for during a film's pre-production process.
Otherwise, romantic chemistry on film may end up like scientific chemistry, creating a poisonous concoction that's harder to keep away from in the tabloid headlines.
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