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.

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!"

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.

Film

 * When Lydia Dietz 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.

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.