Slow Transformation

Something's starting to happen to one of our characters - maybe he's been infected with some virus... Maybe he's going crazy... Maybe he's just gaining weight. Whatever it is, it's usually not directly explained, and we don't see him changing on-screen...

It's just that one day, he looks a little bit... off.

And each time we see him after that, the change is a little more noticeable, usually until it reaches either hilarious or horrific levels.

As mentioned above, this is always played for either drama or laughs.

A Super-Trope to Virus Victim Symptoms, Evil Makeover, and Madness Makeover and a Sister Trope of Transformation At the Speed of Plot.

May also overlap with Metamorphosis, which is often a gradual process.

And Then John Was a Zombie is sometimes the end result.

See Fisher Kingdom (more specifically the second variant) for when simply being in a certain place can have this effect.

Anime and Manga

 * In The Cat Returns, the protagonist becomes more catlike the longer she remains Trapped in Another World.
 * In Simoun everyone is born female and upon turning seventeen go to a mystical spring where they choose to either remain female or become male. For those who choose to become male the process is a Slow Transformation. For example head mechanic Wapōrif went to the spring and became male two years before the series but still has the biggest bust in the series.

Film

 * The protagonist of The Santa Clause slowly changes to fit the part of Santa (gaining weight, becoming unable to shave).
 * The song "The Real Ludmilla" from Bartok the Magnificent has Ludmilla gradually turning into a dragon as the song progresses.
 * The werewolves of the Ginger Snaps films transform slowly into wolflike creatures, over the course of days or weeks.
 * Seth's horrific transformation in The Fly starts out gradually with his fingernails becoming loose and his face having a subtle discoloration to it. Then his body parts begin falling off, starting with his ears, and the Body Horror climaxes with his decaying human skin being torn apart to reveal his true mutant fly form.
 * Wikus' transformation into a Prawn in District 9, to the point where

Literature

 * People turning into donkeys in Pinocchio. In the original story it took several days for the first part of the change (the ears) to occur.
 * The Shadow Over Innsmouth.

Live Action Television

 * There was an episode of The Outer Limits where a person was getting these wierd warts and such. It turned out that a race of reptiloid aliens had sent spies to study humans and the spies were turning back after the disguise wore off. The horror factor was the spies themselves didn't know they were aliens.
 * God!Castiel in Supernatural

New Media

 * Ted Bear's weight gain in the titular Cyanide and Happiness short.
 * Michael Swaim's... Um... "Cuteification" in Vids they Thought Would be Cute.
 * On a similar note, our own page description for Sugar Bowl.

Tabletop RPG

 * Dungeons and Dragons has "artifact transformation"—some artifacts gradually transform their users into "the proper owner" image—physically, mentally or both.
 * WG5 Mordenkainen's Fantastic Adventure. The Kaftan of Ogrishness slowly turns its wearer into an ogre over a period of three days.
 * D3 Vault of the Drow. Anyone possessing the idol of Lolth will, over 66 days, slowly turn into a giant spider.
 * Call of Cthulhu (tabletop game). As Deep One hybrids are growing up they appear to be human. When they reach the appropriate age, they slowly change into Deep Ones and go off to join their fellows.
 * GURPS
 * Space Adventures, adventure "Rebirth". A genetic virus called Proteus can slowly change a human being into an alien.
 * Fantasy Folk. A Fishmen artifact can slowly change its user into a Fishman.
 * Dark Conspiracy. The Darktek supplement had Empathic Viral Mutators, a virus that could change the victim into another creature over a period of 48 hours.

Toys

 * In Bionicle, after the Piraka bathe in mutagenic water for a short while, thanks to falling into a trap, they don't notice it, but they start developing aquatic capabilities. When we next meet them a couple of books later, we find that their whole bodies have diminished, reducing them to eel-like spines with their heads at the end.

Video Games

 * In Guild Wars Nightfall, Varesh transforms into a Margonite like appearence (and eventually a full margonite form) over the course of several missions. This involves getting 2 extra pairs of eyes (first appearing as bumbs, then fully appearing) and become a lot paler.
 * The Wanderer over several cutscenes in Diablo 2 gradually looks more and more evil, until in a final cutscene morphing fully into a Diablo form.
 * In Eternal Darkness one of the characters you play as is inflicted with a curse in their chapter, meaning he gradually rots and turns into an undead creature (this happens at set points in the level) while he tries to to warn Charlemagne before he meets the same fate.
 * The most notable thing about the online retro dungeon-crawling RPG The Haunted Ruins is how
 * The protagonist of Shadow of the Colossus gradually becomes battle-worn as he kills the Colossi.
 * The monsters and face blocks from Eversion undergo this as you gradually evert from World X-1 to World X-8.

Web Comics

 * Faden's Gender Bender curse in Exiern has a nasty side of gradual transformation to it. While the immediate transformation turns his victims female in a blink of an eye, it doesn't stop there. They gradually transform from just basically a female version of themselves to exactly what they picture a stereotypical woman to be. Whether that is a mindless ditz fit only for giggling and bearing children, or a glorified barbie doll.