"Happiness is your dentist telling you it won't hurt and then having him catch his hand in the drill."
— Johnny Carson
Stock Phrase uttered by a character who is about to do something potentially nasty to someone. They tell their victim that what they are about to do won't hurt at all. This often turns out to be a blatant lie.
This is often done by doctors or friends of the patient who genuinely mean well, and are just trying to help them. However, it can also be used as a sarcastic Pre-Mortem/Pre-Asskicking One-Liner by someone who is about to inflict a lot of deliberate pain on their victim.
Another common variant is "This may hurt a little", or alternately, "You might feel a slight tingling sensation". This variation tends to be popular with Mad Scientists. A particularly psychopathic or dickish character may instead say, "I won't feel a thing."
Definitely Truth in Television, as anyone knows who has been to the dentist, although it is increasingly averted as anesthesia gets better and starts to get used more widely. This is also a commonly-used phrase when children who are Afraid of Needles are involved.
Often used to set up False Reassurance.
Mahou Sensei Negima! has Izumi Ako, who claims her needle (which is 1.8 centimeters thick, over a foot long, and inserted in the butt) isn't as painful as it sounds, but some of the members have a different opinion on that.
Hange Zoe in Attack on Titan promises not to hurt the Titan she's fighting...and immediately killed it, saying a variation of this.
In the Naruto fic White Rain, when Rina is ordered to be killed by a Grass genin, said genin uses a poisoned knife and reassures her that it'll be clean and that she'll "barely feel it." However, the genin is reluctant to go through with it, and his hesitation gives Yukio enough time to catch up with them and pull off a Big Damn Heroes moment.
Films — Animated
Robots. After Rodney is assembled, an extra piece is found.
Mr. Copperbottom: [chuckles] We did want a boy, right? [He takes the piece and a hammer] This won't hurt a bit, son. [He hammers the piece in, Rodney cries]
Doctor Allenby: This won't hurt a bit. [Sticks Chance with a needle]
Chance the Gardener: It did hurt.
Chill Factor: An assassin tells one of the protagonists, as she is about to execute him: "Don't worry, I'm a professional, this won't hurt a bit."
Given an Ironic Echo a short while later after he ends up gaining the upper hand: "I'm an amateur, this is going to hurt like hell."
One of the tag lines for Michael Moore's Sicko (a documentary about health care in the US and around the world) is "this may hurt a little". Promotional materials also use "get well soon!"
Bedazzled (1967) - the Devil offers Stanley seven wishes for his soul. Stanley has misgivings and asks "But if you took it out, wouldn't it hurt?" The Devil replies "It's a perfectly painless operation, and afterwards you won't feel a thing!"
Beautifully averted in Hook, where Hook is about to pierce Peter's son's ear and tells him 'Brace yourself lad, because this is REALLY going to hurt.'
Inverted in Creepshow, when Jordy Verrill dreams of what will happen if he goes to the doctor about the growth on his fingers. The doctor tells him the fingers will have to come off, then opens up a steam sterilizer and takes out a meat cleaver. "This is going to be extremely painful, Mr. Verrill," he says.
In The Three Stooges short I Can Hardly Wait, Curly needs to have a tooth extracted at the dentist, and Moe tries to convince Curly he won't feel a thing by sitting in the patient chair and explaining what will happen. The dentist comes in from the other room, quickly puts Moe to sleep with anesthesia, and proceeds to pull out Moe's tooth before Curly can tell him that he's supposed to be the patient.
In Cube Zero, the Cube surgeon at the end falsely assures Wynn that he won't feel anything of the lobotomy they're going to give him. The first thing Wynn does when they cut into his brain is scream out in terror.
In Snow Dogs, this is the response that Dr. Brooks (and later his son Ted) give to dental patients who ask if it'll hurt: "A little. More than a tickle, but less than paying your taxes."
In Enders Game, Ender is told "it won't hurt a bit" to have his monitor taken out, but Ender knows that adults say that when it is going to hurt.
Coraline, the Other Parents tell Coraline this when they try to convince her to let them sew the buttons on her eyes. Coraline doesn't believe them for an instant.
Discworld has the troll retrophrenologist truthfully informing his client "This won't hurt a bit" as he readies the mallet. (Phrenology being the pseudo-science based on determining a person's mental state and personality by measuring the skull and variations thereof. Retrophrenology "works" by introducing new variations to the skull to modify said mental state and personality...)
In The Mysterious Benedict Society, one of the Recruiters tells the children "If you children stay nice and still, I promise this won't hurt a bit." The other Recruiter that is with him suggests "Oh, come on, let's tell the truth for once. Just for kicks." The first Recruiter then admits "All right, the truth is that this will hurt. A lot. But if you hold still, I promise it won't hurt for long."
(River Song is preparing to give Amy a vaccine) River: Now, this won't hurt a bit. (injects Amy) Amy: Ow! River: See? I lied.
In "The Name of the Doctor", the Doctor hooks Clara up to the Tardis's psychic network, via snapping a wire to her hand.
The Doctor: Don't worry, it won't hurt a bit. Clara: Ouch! The Doctor:I lied.
In the mini-episode "The Last Day" a soldier is instructing another soldier in the use of his helmet P.O.V. Cam, which will record everything that happens to him in case he's killed in action.
Soldier: From now on the head-cam is downloading directly to your memory. A tiny part of your brain is now a hard-drive. Takes up hardly any space; you won't feel a thing. Now, at the beginning sometimes this has side effects— (cut to hallucination of a screaming soldier)
Inverted in the prequel episode "The Night of the Doctor". The Eighth Doctor is about to take a potion which will make him regenerate into the War Doctor.
The Doctor: Will it hurt? Ohila: Yes. The Doctor:Good.
Hyperion Spy: "I'm starving! I wanna eat your babies!"
Zed: "Son, this might sting a little." *violently stabs Hyperion jerkbag with gigantic needle*
Friendly Hostility: Charlotte, a Mad Scientist in training, uses her father's employee Derringer as a guinea pig for a machine labelled "Schroedinger's Derringer". "Now in a moment you may or may not feel a tingly death ..."
Doc McStuffins: The whole clinic staff uttered this exact phrase to an uneasy Niles while trying to get his bandages off. Of course, Niles was expecting it to hurt. Inverted in that they proceeded to carefully take off most of the bandages while Niles wasn't paying attention, so he naturally felt nothing when Doc told him it was time to take them off and pulled a little piece of bandage still on him.
From the obscure and long-since-canceled G4 series Portal (no relation to the game): "I was going to keep you as my personal music slave! Now I think I'll pull your stomach out and wear it as a man-purse! Don't worry, it'll only hurt for thirty minutes!"
Jimmy Neutron is usually asked this whenever he tests his inventions on his friends.
Carl: Is this gonna hurt?
Jimmy: Oh no, I'll be fine.
Buzz Lightyear of Star Command has an extended while introducing Torque as the Villain of the Week: Zurg, seeing that Torque shares a hatred of Buzz but also lacks the method to act with it, decides to tweak his genetics a little bit. The catch (of which Zurg is fond) is that the procedure to "power-up" Torque is going to hurt. Very hurt. Very very hurt. Zurg warns Torque just before the operating machinery begins its work, however, the machinery has already been activated and operates very loudly, and Torque can't hear him:
Zurg(as the noise of the operating machine reaches a peak): Did I mention the operation will be excrutiatingly painful?" Torque: What was that, Z? Zurg: Oh . . . ah, nevermind! (giggles)
Zurg exits the operating room and closes the door as we hear Torque begin to scream.