Intimate 'Challengers' scenes set the internet ablaze. So, what makes for a great movie kiss?

Challengers might be a movie about tennis, but the steamy kissing scenes involving its three stars are what have viewers transfixed.

There’s no onscreen sex to speak of in the film, but the characters portrayed by Zendaya, Josh O’Connor and Mike Faist share several passionate make out sessions. That includes a now-infamous three-way kiss scene made into videos and GIFs and shared online countless times before the movie even left theaters.

“It’s beautiful to kiss people! That’s what I want to say,” director Luca Guadagnino told Variety about the scene. “People, kiss! Do not make war.”

Throughout cinema history, a number of movies have been immortalized for their iconic kissing scenes. The Notebook features a zealous kiss in the rain between Ryan Gosling and Rachel McAdams, made even more notorious by rumors that the two didn’t get along on set. When Cary Elwes kissed Robin Wright in The Princess Bride, the movie’s narrator declared it among the five most “passionate” and “pure” of all time. Who could forget the “kiss me as if it were the last time” scene between Ingrid Bergman and Humphrey Bogart in Casablanca?

So, what makes a great movie kiss — one between actors that is memorable, passionate and believable?

Maria San Filippo, a professor at Emerson College and author of Provocauteurs and Provocations: Screening Sex in 21st Century Media, told Yahoo Entertainment that some of the kissing scenes that were most memorable to her had interesting choreography and cinematography.

She said a kiss between Cary Grant and Ingrid Bergman in Alfred Hitchcock’s Notorious came to mind for its “sheer length” and the way the actors “manage to carry on an entire conversation” while kissing.

“Another famous one from the classical era also by Hitchcock is the 360-degree shot around the kissing couple in Vertigo that visually conceived the trope of the world fading away as the lovers embrace,” she said. “It spawned many later imitators, such as the final clinch in Never Been Kissed, yet with far more troubling overtones in Vertigo.”

San Filippo noted that some might think it sounds “contradictory” to make a choreographed scene between two consenting partners look “in the moment,” but she sees it as “the power of performance.”

Much of what goes into a good onscreen kiss happens behind the scenes. Challengers star O’Connor told Yahoo Entertainment that an intimacy coordinator was heavily involved while filming the movie, and director Guadagnino had a clear vision of what he wanted. O’Connor and Zendaya both mentioned that the intimate scenes felt choreographed “like a dance.”

Chelsea Pace, an intimacy choreographer and coordinator who wrote the book Staging Sex: Best Practices, Tools, and Techniques for Theatrical Intimacy, works to make sure actors feel safe and informed while filming intimate scenes. What exactly that entails varies broadly from one actor to another, and might change over the course of the process.

“Some artists feel supported by a quick conversation followed by a thumbs up between takes, but others prefer … developing choreography with a scene partner, director and intimacy coordinator,” she explained. She has ongoing conversations with everyone involved about what they’d like to see and what they feel comfortable doing, and those begin long before the camera is rolling.

“Actors don’t always have the same intimate experience as their characters,” Pace said. “Maybe they’ve made out before, but they haven’t made out with two people in the midst of a tennis tournament.”

She encourages actors to pull away from their “lived intimate experience” to tell a story. Before intimacy coordinators were popular, actors would be given instructions and expected to decode them. Now she works with actors and their directors to figure out exactly what that looks like.

“I am an expert in intimate physical storytelling,” she said. “I can look at a scene and say, ‘Oh, this doesn’t look right because their weight is on their left elbow. Make this little shift and we’ll tell the story better.’”

When it comes to the actual mechanics involved in making an onscreen kiss look good, Pace sometimes offers actors tips. One of her favorites is suggesting they inhale through the nose as their lips make contact with a partner, and exhale as they pull away.

“It creates this sense that people are breathing hard together or gasping for breath,” she said. “It also creates this really great line on your neck.”

Many movies include kissing scenes that are well performed, directed and choreographed. What sets Challengers apart from others is its authenticity. In addition to the three-way kiss, there are several other notable kissing scenes: In a dorm room before a tennis match, outside of a car in an Applebee’s parking lot and outside in the middle of a wind storm.

Carrie Wittmer, a senior features editor for L’Officiel USA who has written about the art of onscreen romance, told Yahoo Entertainment that the kissing in Challengers is particularly good because “they allow it to be disgusting.”

“Kissing is romantic, but if you think about it and look about it, it’s gross,” she explained. "Kissing sound effects should sound nasty, like stirring a thick, hot bowl of freshly baked macaroni and cheese."

Tons of choreography and camera work may be necessary to bring a vision to life, but if it’s too obvious, it feels “jilted and strange,” she said.

Challengers doesn’t have that problem.