It's Showtime!

A Stock Phrase and a specific instance of Pre-Ass-Kicking One-Liner, "It's Showtime!" (or just "Showtime") is used often enough in media to count as a trope on its own. Its utterance by a character usually heralds the time to get dangerous, or the moment when a deception or disguise is about to be critically tested.

Naturally, this trope excludes instances where someone is announcing that an actual performance is about to start.

Not to be confused with the Philippine noontime variety show or the 1976 documentary film of the same name.

Examples of It's Showtime! include:

Anime and Manga

Roger Smith summons his Humongous Mecha The Big O by activating his remote-control watch and bellowing "Big O! Showtime!"
- Late in the series, when Alex Rosewater activates his mecha, he calls out "Big Fau! Showtime!"
Sengoku Basara: Date Masamune's Gratuitous English bellow of "Showtime!" is an obvious instance.

Fan Works

The first time metahuman Douglas Sangnoir goes into battle at the side of the Knight Sabers in Drunkard's Walk II, he declares "It's showtime!" to himself.
Chapter 23 of Princess of the Blacks by "Silently Watches" ends with the title character's thoughts at the very start of the Triwizard Tournament's Yule Ball:

Let's see; get through dinner and an evening of dancing without allowing anyone to see through the illusion, confront an incubus, and avoid making a fool of myself in a social situation I have never experienced before. Piece of cake. She took a subtle breath as she linked arms with her escort, letting it out when the champions began moving forwards.
Showtime.

At the end of Violence Inherent in the System, part two of The Girl Who Loved, the narrative voice says "Showtime" to start the spectacle of Cuteness returning to the future.
Kaji says this in chapter 13 of the Neon Genesis Evangelion Peggy Sue fic Once More with Feeling as he and Shinji, having completed their debrief with the JSSDF and returned to the mall, are about to rejoin Misato, Asuka and Rei.
In her role as the first-person Narrator of the Veronica Mars/Buffy the Vampire Slayer crossover fic Martian Manhunter, Veronica ends chapter 85 (and its preparations for an assault on Spike and Drusilla) with the single-word paragraph "Showtime."

Film

When Lydia Deetz finally invokes him at the climax of Beetlejuice, Betelguese looks up from the miniature headstone on which he's been sitting, grins, and growls, "It's showtime."
Matt LeBlanc as Major Don West in the 1998 Lost in Space movie declares "Showtime" when it comes time to launch the Jupiter II.
Bob "Mr. Incredible" Parr says "Showtime" at multiple points in The Incredibles, always just before launching into superheroics.
In the 1991 The Addams Family film, Gomez shouts "Showtime!" when he finds the home movie reel he's looking for in a box in the vault.
"It's showtime, folks!" is Rob Gideon's Catch Phrase in All That Jazz.

Live-Action TV

Jimmy McGill quotes All That Jazz repeatedly in the final season of Better Call Saul.

Video Games

In Marvel vs. Capcom 3: Fate of Two Worlds, the Black Cat declares "Showtime!" to the consternation of Spider-Man, who claims it as his line.

Web Comics

Girl Genius had "SHOWTIME!" as the activation word for combat mode of Agatha's clanks in the arc "Agatha Flips Out (Big Fight)".

Western Animation

On an episode of The Simpsons, Mr. Burns and Homer are trapped in a snowed-in cabin thanks to an avalanche, caused by them. The two begun to crazy and start a fight. After Mr. Burns present an army of snowmen, Homer presents random political figures, with Abe Lincoln shouting this line.