There's an urban legend in television circles about a couple on a soap opera who got so carried away during their love scene that they didn't hear the director call "Cut." Anyone who has ever filmed a sex scene knows the above story is a myth: what the audience sees on the screen is a far cry from the actual production of the scene.
First, you've got the crew -- up to 80 people standing around watching you, naked, simulate making love to an actor you may or may not know. It's not unheard of for sex scenes to be filmed on the first day of production, which means you're nearly naked (save for a flesh-colored thong or pasty) in front of dozens of strangers, trying to make your body and mouth move the way the director wants, trying to make it look like you and this total stranger are having the best sex anyone has ever had anywhere.
After each take, the makeup department runs over touch up your lipstick, mascara and powder, then someone else comes and spritzes you with rosewater and glycerin (to make it look like you've been sweating passionately); meanwhile, a production assistant replaces the pillowcase (lipstick and mascara smears), and the director is giving the cameraman notes ("Try to get her thighs from a different angle - her cellulite is wrecking the whole scene"). And, if you're extremely unlucky, your co-star takes this opportunity to smoke a cigarette, chew some raw garlic or gargle with vinegar.
The majority of directors are notoriously precise when it comes to choreographing sex scenes. "Put your left leg over his right thigh, then you -" pointing at the male actor -- "You roll her over on her left hip..." The entire thing becomes like a high-budget game of Twister. Then, of course, you have to hold those positions while everyone checks to make sure the camera angles are correct, the makeup still looks good, the sheets haven't been sullied... and it's "Action" again. Five, ten, fifteen, twenty times.
In order for a camera to focus on the lips touching, the faces need to be farther apart than they are when people are making out in real life. In most cases, unless the director specifies, there's no tongue involved. It's considered rude. Many actors employ a "lips open, teeth together" philosophy: it may look like hot 'n heavy kissing, but what you're seeing is lip-acting of the highest caliber.
Adding another layer of weirdness to the whole sex-scene thing is ADR, or Automatic Dialogue Replacement. Ever notice how, in the middle of a love scene, the audio suddenly becomes louder? It's difficult to mike two people who are lip-locked; frequently, they must 'voice-over' their passion by standing alone in a recording booth, wearing a headset, watching the scene soundlessly, and adding their own "ooh baby" and "oh yes" - sometimes several months after the scene was originally shot.
So the next time you're watching 9 1/2 Weeks, The Thomas Crown Affair or whatever you think is the sexiest movie ever made, take a moment to appreciate just how much acting is going into those steamy scenes. 
-- Sarah Chauncey
