It's one of the most intimate exchanges known to mankind - so intimate that even prostitutes refuse to do it.
But Hollywood stars are happy to lock lips for all to see, and actors and actresses even boast and joke about their screen kisses.
Some make a joke of their smooching, others reveal how awkward kissing a stranger can be, and some go to great lengths to make the experience as far removed as possible from the romantic moment it's supposed to be.
The screen kiss was once the stuff of Hollywood legend, with crew and cast sworn to secrecy about what really went on ? but nowadays the movie stars are happy to talk about their kissing time, and what they do to ease the tension of a stolen smooch.
Even movie veterans are coming forward with their memories of classic kissing scenes, which were closely guarded secrets back in the day - the stuff of taboo.
Take Tony Curtis, for instance, who has recently revealed what it was really like to kiss screen icon Marilyn Monroe in Some Like It Hot.
The couple was an item at the time, so the kiss moviegoers saw on the big screen back in 1959 was very much for real.
Curtis recalls, "I've been misquoted as saying kissing Marilyn was like kissing Adolf Hitler. That's nonsense.
"This big beautiful blonde lay on top of me for five hours a day for a full week - I was in heaven. After Marilyn had given me a real big kiss, she would say, 'How did you like that one, big boy?' and I would somehow manage to croak, 'Marvellous.'"
That was then, but the loose lips have really started chatting about screen kisses in the past few years.
The 'What was it like to kiss...?' question is often the first thing stars are asked when they're promoting a hot film with a love scene or two.
And most are more than happy to reveal all - from how they prepared for the smooch and whether it was any good to their worst nightmares.
Julia Roberts is perhaps the queen of the screen kiss - she always seems to be locking lips with a leading man.
In American Sweethearts it was with John Cusack, and then immediately afterwards it was with Blair Underwood in Full Frontal.
Cusack and Julia were so concerned about bad breath during their early morning kissing scene that they were lampooned in the film - with Julia's character Kiki covering her mouth with a bed sheet.
She recalls, "You wake up and your mouth tastes terrible. I'm always worried about my breath - and it's not that I even have bad breath.
"I call it sleepmouth or bedmouth - when you wake up and you know you can't kiss. It's there in the film for all to see. Thankfully, neither John nor I really suffer - but you do worry."
But Julia's kissing scene with Full Front al co-star Blair Underwood was particularly tough - because her husband-to-be Danny Moder was on the other side of the camera.
Underwood insists the cameraman was the perfect gentleman, but Roberts admits it was one of her most awkward moments.
She recalls, "I was losing it. I've never been in that situation before and just thinking about it now I can feel the flames licking my chain.
"It's bizarre enough when you say, 'Hi, nice to meet you,' and you kiss, but, 'Hi, nice to meet you,' and here's the rest of your life sitting right in front of you, and kiss. It was weird.
"The dudes on the set were cool. I was the one laughing nervously."
Underwood admits the awkward scene was difficult for him too.
He admits, "You're doing a scene and you're kissing an actress and the actress' boyfriend is standing right behind the camera. It's always an issue, man.
"I felt like he was right next to the camera saying, 'Don't put the tongue in. No, no, no.'"
Will Smith has also fared pretty well in the kissing stakes of late - smooching with his wife Jada Pinkett Smith and Nona Gaye in Ali, and with Rosario Dawson in Men In Black II.
The movie star admits kissing his wife was really special - because they pretended they were different people.
He says, "I was Ali for that kiss and Jada was Ali's first wife Sonji so we got a bit of role play going. It was very sexy."
Jada adds, "It was really romantic playing these two people on the set and then being at home together as man and wife. With the kids not there on set, if felt like we were doing something sexy. The sparks flew. Will's a great kisser."
Rosario Dawson couldn't agree more - although she understands what Jada means about kissing in front of the children. She has to pucker up while Will's mischievous son Trey watched.
She recalls, "It was Jada's birthday and she was on the set watching me kiss her husband.
"That's bad enough but Trey was like, 'Ooh Jada, I can't believe you're gonna let your man kiss another girl on your birthday.' It was a tough scene."
And then there are the stars who like to make the kissing scene as unromantic as possible - and even disgusting!
Dallas star Linda Gray, who played Sue Ellen in the hit soap, has recently revealed she used to fill her mouth with mushy peanut butter for kissing scenes with her JR, Larry Hagman, and it seems Gray's disgusting prank set the standard for today's screen kissers.
Britney Spears' Crossroads co-star Anson Mount made light of her first screen kiss by munching on garlic before locking lips with her.
He giggles, "After we kissed a couple of times, I took a big spoonful of minced garlic and washed it around real good.
"When we did the scene, Brit amazingly kept a straight face. After they yelled, 'Cut,' I asked her, 'Did you taste that?' and she goes, 'Oh sweetie, I just didn't want to hurt your feelings.'"
Britney herself admits that, garlic breath aside, Mount was a good kisser: "Having all the cameras around me at first was kind of nerve-racking but he made me feel comfortable and it was totally fine. He was a sweetheart."
And she isn't the only one who enjoyed her stolen movie kisses, there're plenty of actresses who would gladly lock lips with a former film lover.
Sultry Rene Russo would never pass up the chance to smooch with her Thomas Crown Affair leading man Pierce Brosnan - he certainly has the golden touch when it comes to kissing.
Russo reveals, "Pierce is the best. He's an absolutely great kisser.
"He kissed me like I like to be kissed - soft and hard at the same time."
Russo was also very fond of Mel Gibson when she kissed him in Lethal Weapon 3.
She adds, "He and Pierce are very similar men - a lot of charm and a genuine liking for women. That comes off when you have to do a kissing scene. They both look like they know what they're doing."
Mel has another fan in his What Women Want co-star Helen Hunt. The Oscar winner admits she was worried about having to kiss a man who was a chain smoker at the time.
But when it came to the scene, he could have been the foulest smelling man in the world - he was still Mel Gibson.
Hunt says, "Was it tough kissing Mel after he'd just smoked a cigarette? No. Who cares? A little thing like that can't make Mel Gibson a bad kisser! No, it was a pleasure.
"He's certainly a fantastic kisser in my opinion. For him? I don't know. He may have been looking at his watch every five minutes. Mel is very perceptive about women. I found him open and willing to listen and curious. That's a very sexy quality, if someone's curious and really wants to know what you think.
"He's the greatest guy in the world. A really great kisser."
Juliette Lewis was delighted when she learned that she had to kiss Robert De Niro in thriller Cape Fear, but her teenage crush turned into an obsession shortly before their intimate scene.
The then 18-year-old Lewis admits the kissing scene was so special because De Niro was so professional and caring.
She recalls, "I was like, 'Should I use mouthwash? No, because then he'd know that I care.' I wanted to be, 'Oh, it's cool, he's going to kiss me, whatever. Hadn't even thought about it.' But I was a wreck.
"I walked on the set and I smelled this minty fresh aroma coming from Mr De Niro and it dawned on me that that's professionalism. It was a lovely experience. Whenever I get down I can always say to myself, 'I've kissed De Niro.'"
There are two new top kissers amongst the ranks of Hollywood's leading men, in the shape of Vin Diesel and ,Matt Damon.
Diesel's XXX love interest Asia Argento admits the tough New Yorker has the softest lips ever and she'll never turn down the chance to smooch with him on screen again.
The sultry Italian says, "I imagine Vin's a great lover - strong but tender. He is a terrific kisser. I'm kinda sorry our characters didn't go further."
It's a good job for Vin really, as Asia actually had sex for real in her new film Scarlet Diva - to capture the pre-climax face of her character.
And Matt Damon has been given kissing kudos by his Bourne Identity co-star Franka Potente.
The German actress was very impressed with Matt's technique and spoke so much about his kissing abilities that gossips were sure the two were having an affair.
She says, "We were kissing all day. I brought my toothbrush and my mints and whatever. Matt's a great kisser."
Of course, not all screen kisses are pleasant experiences.
Funnily enough, there are very few actors who will bitch about a bad leading lady, but there're plenty of actresses who will kiss and dismiss their leading men.
Take Helena Bonham Carter, for instance, who admits kissing Woody Allen in Mighty Aphrodite was "repulsive".
She explains, "He tells you up front certain ways of kissing he does not want. No exchange of liquid is permitted. It can be a bit offensive because he makes no effort at all.
"He does warn you, and says everyone gets the same treatment. His mouth is a no go area. It's like kissing the Berlin Wall."
And even the hunks come in for hard criticism when it comes to kissing scenes.
Leonardo DiCaprio was convinced he had done a pretty good job of kissing French movie star Virginie Ledoyen underwater in The Beach, even though it was a far from romantic experience: "I had to hold Virginie down and she had to give me hand signals for when she was out of air."
But Ledoyen was far from impressed with the Titanic star.
She reveals, "I think Leonardo is a nice guy. But I didn't want him as a lover. I don't like him that way at all.
"Kissing an actor as part of the script is nothing like being in bed with a real lover. There is no honest passion. No real sensitivity in our love scenes.
"In our underwater love scenes all I could think of was not drowning. You can't breathe. I can't really remember his kiss."
Kissing gay actor Rupert Everett was a "weird" experience for ,Madonna in The Next Best Thing.
"It was a bit weird but you just gotta do what you gotta do. If I didn't know him probably it would have been easier because having known him for so long, It's kind of kissing your brother."
Star Wars star Natalie Portman knows just how she feels - she felt terrible after kissing James Frain in her first smooch role Where The Heart Is.
She recalls, "I just felt so weird being told to kiss someone I didn't want to kiss. I felt almost whorish, being told what to do in a sexual way.
"Thankfully, I didn't have a boyfriend at the time - can you imagine how terrible that would be for him."
And British actress Thandie Newton has brought Tom Cruise down a peg or two after smooching with him in Mission Impossible 2.
Cruise might be rethinking his choice - he handpicked Thandie to play his love interest in the film.
But she admits, "Kissing Tom Cruise was slightly icky and sort of wet.
"I'd really go home at the end of the day actually moaning about how hot it was and how many times we had to do it.
"To be fair, it's not really about Tom. Kissing scenes are just tough - you can't kiss too hard because your face spreads across the other person, which is terrible.
"And then you don't want to get too much make-up running all over them, so it's so clinical. You find that you're not really thinking about kissing. You're thinking about, 'Is my head in the right place? Am I squashing him?? It just takes all the intimacy out of it."
Even Brad Pitt doesn't hit the grade as a great kisser - just ask his Interview With The Vampire co-star Kirsten Dunst.
The young actress had to pucker up for a scene with the actor when she was only 11, and admits that it was not enjoyable at the time.
She says, "I am sure I would love to do it now, but back then, I was 11 and it was 'ugh'. There's nothing sexual or sexy when you're that age.
"So I was kissing Brad Pitt. So what? He had chapped lips. He was lovely and kind and sweet to me, but it was just yuk."
Meanwhile, Kate Winslet has had her fair share of big screen kisses in romantic films like Jude and Titanic, but there was one smooching scene she didn't enjoy.
Kate had to pucker up for a lesbian kiss in Holy Smoke - and admits she had to get drunk to do it.
She admits, "I wasn't drunk to the point of falling over, but it really helped me to find out what I needed to do for the scene."
Lucy Liu and Calista Flockhart had exactly the opposite feelings about their lesbian smooch in Ally McBeal.
"Our kiss was a truly intimate moment and I am glad television was able to air it," Lucy says.