Digital Head Swap

Relatively recent among filmmaking effects because it's very difficult to do without CGI, this is a technique that makes it possible for an actor to be uninhibited by the limitations of his body type. It's mostly used for comedic purposes, creating a sort of Mix and Match Critter situation with a man's head on a woman's body, or something similarly ridiculous. Also popular is the method of putting a real face on an otherwise CGI character or vice versa. It can also be done to hide the use of stunt doubles or just to fix a shot in which someone's face is obscured or otherwise not ideal for the film.

Done poorly, especially in early cases when the technique was not perfected, this can easily create Conspicuous CGI and dip into the Uncanny Valley, although this effect can also be invoked on purpose.

Nothing to do with Head Swap, which is a Video Game technique to increase the number of characters without drawing as many sprites.

Special Effects Examples:

 * Possibly the Trope Codifier for the digital age is Jurassic Park, which did this out of necessity. During an action scene, a stunt double who was suspended over a mat (which later became a raptor's mouth) looked directly up at the camera. Normally this would have ruined the shot, but the effects team were able to splice the normal actress' face over that of the double, turning an otherwise unusable piece of film into a particularly heart-stopping moment.
 * This same technique was used in Titanic when Jack and Rose are running down a flooded hallway. A bit more noticeable though as the sequence was longer and in slow motion.
 * In The Matrix Reloaded, Hugo Weaving's head was digitally overlaid on the bodies of the stunt double "Agent Smiths" in the Burly Brawl sequence.
 * In Tim Burton's Alice in Wonderland, Tweedledee and Tweedledum have actors' heads on digital bodies. As well, the Red Queen's head is greatly enlarged.
 * Done for Guru Pitka's child self (with Michael Myers' normal head) in The Love Guru.
 * The Wayans brothers vehicle Little Man, wherein one of the brothers took position on the body of a child actor.
 * The Adam Sandler vehicle Click, during the segment in which his character was enormously fat.
 * Jack and Jill (2011), also with Adam Sandler, this time playing his own sister.
 * This was going to be done on Captain America: The First Avenger for the scenes of Steve Rogers before he becomes Cap, sticking Chris Evans' face on a shorter, scrawnier stand-in. The director decided against it, in order to preserve Evans' unique body language. Instead, Evans was digitally shrunk down.
 * Done in The Social Network with the Winklevoss twins, who were played by two different actors with the first actor's head placed on the other's body.
 * Done to eerie effect in the video for "Weird Al" Yankovic's "Perform This Way", a parody of Lady Gaga which put Al's head on a woman's body.
 * The video for "CNR" is made with enlarged footage of real faces spliced onto bodies which are a mix of filmed actors and other animation techniques, creating the effect of a world in which everyone has giant heads and mostly fixed expressions.
 * The Crow: Brandon Lee's face was digitally grafted onto a stunt performer's body for a small smattering of scenes not yet shot at the time of his death.
 * Blade Runner: The original version had a shot during Zhora's death where it was obvious that a stunt double (more specifically, the awful cheap wig they had to use) was standing in for the actress. For the 2007 Final Cut, actress Joanna Cassidy's face was digitally superimposed over that of the stunt double.
 * There's a bit on Late Night With Jimmy Fallon titled "Head Swap", which is introduced as this trope, but the theme song gets derailed into a Random Events Plot that takes up the entire segment. See an example here.
 * Used to rather disturbing effect in an Aphex Twin video, putting his bearded face on several bikini-clad girls.
 * Done in an episode of That 70s Show, where after getting high, the gang sees Kitty's and Red's heads on each other's bodies.
 * The film Beowulf did this, after a fashion, for the lead actor, Ray Winstone. The whole thing was filmed using motion capture, and Winstone claimed they put his head on his eighteen-year-old body. The same sort of thing was done with Angelina Jolie.
 * Recent commercials for Orville Redenbacher's popcorn featured the company's namesake CGI'd post-mortem onto a real actor's body to creepy effect.
 * Star Wars: In the new series, this was done with Count Dooku and Palpatine for their lightsaber battles, quite necessarily given the ages of their actors Christopher Lee and Ian McDiarmid. However, the quality of the effect has resulted in Uncanny Valley for some.
 * An early non-CGI example in the movie Bachelor Party, where Tom Hanks's character is conflicted about sleeping with another woman before his wedding and sees several heads of his wife, his male friends, and a nun, superimposed upon the girl's naked torso, exhorting him to either do it or not, Floating Advice Reminder style.
 * The George Lopez Show did this incredibly obviously in its flashbacks, with the adult actors' heads pasted onto child actors' bodies.
 * The video for the Basement Jaxx song "Where's Your Head At?" has the band members faces superimposed onto the heads of monkeys; the plot being about a mad scientists plan to make a monkey band.
 * In the video for the Aerosmith song "Pink" the band member's faces are put on various random people's bodies.
 * A rather impressive example in Tron: Legacy where Jeff Bridges's 20-year-younger head is digitally re-created. Jeff Bridges performed motion capture for the younger CG face, but another actor entirely filled the glowy shoes.
 * Kung Pow: Enter The Fist is probably the most spectacular use of this trope, and certainly the most ridiculous. Steve Oedekerk took a 1970's Kung Fu movie and used this technique to composite his head over the main character's, essentially pulling a Digital Head Swap for at least half of the movie. And then he dubbed every character's voice, including his own.
 * ...and on the exact opposite of the spectrum, we have Game of Death. In order to complete the movie after the death of Bruce Lee, they hired FakeShemps to stand in for Lee by wearing masks made of cardboard cut-outs of Bruce Lee's face.
 * In The Lord of the Rings, this was one of the techniques used to create the illusion of small Hobbits, with actors' faces placed digitally (and sometimes, in wide shots, with just masks) over smaller doubles.
 * Malkovic Malkovic Malkovic: Malkovic Malkovic Malkovic Malkovic Malkovic, Malkovic Malkovic.

In-Universe Examples:

 * Done in-universe in The Running Man movie. Since the heroes cannot be found, the producers of the evil game contest use stunt doubles for the final fight and digitally put the heroes faces on them.
 * Done in-universe in Starship Troopers 3: Marauder, Sky Marshal Omar Anoke is lost, and Admiral Enola Phid is able to make it appear that Omar Anoke is still alive and well in the Federation using computers to put Anoke's image over her on a FedNet broadcast.
 * In Rising Sun, a computer programmer notices that the head of an alleged murderer caught on video has a blue halo, suggesting that his face had been digitally inserted. She demonstrates the technology on Sean Connery's and Wesley Snipes' characters, swapping their heads in the computer.
 * Sliders has an in-universe example (the show was made before this became commonplace); in a universe where criminals are tried and executed live on television, a corrupt TV host killed someone on camera and then framed another guy by editing his head onto his own body.