Beauty Inversion

An actress known to be good-looking who plays a role which calls for her to indulge in a makeup that makes her look homely usually gets flattering notices from the reviewers, even if her performance isn't worth a nickel[.]

—George Jean Nathan

An actor or actress known for their physical beauty being altered by makeup and prosthetics to play an unattractive or even ugly character, usually as Oscar Bait. This doesn't cover really deformed or monstrous characters like Freddy Krueger or Quasimodo; that would be The Grotesque.

Note: This is different from Hollywood Homely. In Hollywood Homely, an actor or actress will be "plained down" with unflattering clothing or hairstyle, glasses, a better-looking sibling, but the result is only (maybe) average-looking by non-Hollywood standards. This also (intentionally or not) leaves an opening for a Beautiful All Along reveal. It also covers characters who might be objectively attractive, but hindered due to crippling shyness or other social issues. This trope is when the actor uses makeup or prosthetics to change the look of their face to match the expectations and image of the character.

This trope usually happens in biopics and serious dramas, and covers attractive actors playing below average-looking characters (or real people), and there is a definite effort to not only play down but deliberately counteract the actor's usual good looks. Since most Hollywood actors tend to be very attractive, it's an interesting challenge for them to portray a Gonk.

If they're playing outright monsters who are usually portrayed as ugly, this may result in a Gorgeous Gorgon.

See also Dyeing for Your Art, which generally covers actors who change their appearance for their roles. Contrast with Adaptational Attractiveness.

Examples of Beauty Inversion include:

Comic Books

In-universe, unusually justified example: In Passionella by Jules Feiffer, Passionella, having achieved success as a movie star, decides to make a more realistic movie where she plays a non-beautiful, unglamorous, chimney sweep. All of her scenes in the movie are shot during the day, where all her previous glamorous roles could only be filmed at night, because she only transforms into a beautiful, glamorous movie star in the evening. She wins the Best Actress Oscar, of course.

Film

Charlize Theron as Aileen Wuornos in Monster. The actress gained between 25 and 30 pounds, shaved her eyebrows, and wore prosthetic teeth. As such, she looked completely unrecognizable in character and got the Best Actress Oscar in a walk.
Salma Hayek as Frida Kahlo in Frida. To play Kahlo, Hayek either used make-up or grew out her own mustache and unibrow.
Nicole Kidman wore a prosthetic nose to play Virginia Woolf in The Hours.
Emma Thompson in Nanny McPhee.
Tom Cruise wears a baldcap and a fatsuit in Tropic Thunder.
Cameron Diaz in Being John Malkovich wears frumpy clothes and frizzy hair.
Brad Pitt at his homeliest was in 12 Monkeys, I think—didn't use prosthetics, but gave the character a lazy eye and unflattering hairstyle.
Gwyneth Paltrow and Brooke Shields in Shallow Hal. When the movie got flak for casting such a thin woman as a heavy character, the director defended himself by noting that the concept called for her to be attractive through most of the movie—Paltrow could fat up while a heavy actress could not thin down. The result is a movie littered with bad fat jokes... about the smoking hot Paltrow.
Almost all the women in the movie version of Push. Mariah Carey (Precious' social worker, pictured above) is the most obvious example, but the lead actress Gabourey Sidibe was also made to look larger using unflattering clothes.
Sharon Stone in Alpha Dog.
Johnny Depp had his head shaved (by Hunter S. Thompson) and wore huge glasses in Terry Gilliam's Fear and Loathing in Las Vegas. Benicio Del Toro just put up a lot of weight and a leatherman's moustache. They both looked quite unspectacular in result. But it was the brilliant acting that made them truly offputting.
Heath Ledger as the Joker in The Dark Knight, although YMMV.
Jude Law played a slimy, gaunt, brown-toothed, heavily balding photographer/assassin in Road to Perdition with disfiguring facial scars in the end.
Gloria Swanson in Sunset Boulevard had to look more aged as Norma Desmond than she did in Real Life.
The handsome, elegant Dame Judi Dench playing the haggard spinster Barbara Covett in Notes On A Scandal.
This is Elizabeth Banks. This is Elizabeth Banks as Effie Trinket in The Hunger Games, under ten tons of makeup to bring out her wrinkles and make her look more like Joan Rivers than a 37-year-old woman has any right to.
Blake Lively in The Town looks convincingly unglamorous and disheveled, sporting ratty clothes, cheap makeup, and noticeable cuts and bruises on her body looking about as far removed from Gossip Girl as its possible to get.
Jude Law in Contagion has snaggle-teeth to point out that he's the bad guy.

Live Action TV

Big Rhonda on That '70s Show. The actress who plays her appeared out of costume in a flash forward once, and looked nothing like Big Rhonda.
America Ferrera as Ugly Betty began this way, before the costume department turned her into Hollywood Homely two seasons in.
Ana Maria Orozco, the original Betty in Yo Soy Betty, la Fea, definitely committed to this trope: no makeup, terrible hairstyling, huge ugly glasses, horrible clothes, orthodontics, the most awkward body language possible… The producers even hired an expert to choose specifically the most unflattering colors and styles to dress her, then the writers incorporated that knowledge on the script to justify and explain her makeover and her business plan.
Amy Sedaris is another comedian who is attractive in real life, yet prefers playing unattractive characters. The David Sedaris essay "Shiner Like A Diamond" noted how she is easily the most attractive member of the family, yet all her life she enjoyed hiding it under wigs and prosthetics. She has said in interviews as well that she's never comfortable doing straight-up sexy, she can't help but add some sort of goofy element to it.
- Don't forget her "fatty suit".
Helena Bonham Carter as Morgan in Merlin. In the first act she is a homely girl with a lazy eye and pigtails, but uses magic to make herself beautiful in order to trip up Arthur. At the end her magic slips away and she is homely again.
Angela Bettis as the title character in the TV remake of Carrie. Drab clothes, messy hair and something about not washing her face or anything definitely hid any beauty that she had, making it that much more special when she went to the prom. Compare with the film versions, where Sissy Spacek in the 1976 film was merely styled plain on her scenes before the prom but acknowledged in universe as “pretty if you took care of yourself ”, while Chloë Grace Moretz in the 2013 film was just too beautiful and no real attempt to make her plainer was made.

Theatre

Any actress who plays the Witch in Into the Woods until the end of the first act. The ultimate example may be Bernadette Peters from the original Broadway production.