Mistaken for Granite

"These statues were singularly inanimate. I knew better than to assume that they would remain as such."

—Kain, Legacy of Kain

Obviously, guarding the MacGuffin is a hard job, especially if you are a 4000 year old civilization. Booby traps can be dodged, guards require resources, and curses can be awesome. So why not just use the obligatory scary statues as guards??

This trope refers to times when an important room has statues around it which seem like normal statues, if perhaps heavily armed. However, upon something important happening, the statues will come to life and start attacking. If the work in question is a video game, the statues will most likely respond to an attack. One way to gain these for your room is to take people for granite.

Examples of Mistaken for Granite include:

Anime and Manga

Inuyasha: a pair of such statues guard the doorway to the underworld.

Comic Books

Miracleman: The doors to the room housing the kingqueen of the Qys is guarded by two guards whom Miracleman/Marvelman mistakes for statues, due to their immobility and size.

Film

Night at the Museum has the giant statues of Anubis that guard the pharaoh.
Mighty Morphin Power Rangers have the statues that guard the Great Power.

Literature

In Conan the Buccaneer the shrine of Tsathoggua the Toad God contains a stone statue of the god, watching over the treasure. If someone step around the temple, the statue comes back to life (still remaining stone) and chase them.
- On a similar note we have the Bloody God (which in this case is a living statue made of gold and rubies).
Subverted in Shadow Keep: at one point the heroes walks in a corridor full of mean-looking, demonic statues. This being Shadowkeep, Sranul thinks that the statues will come to life and attack them, but they're eventually revealed to be just harmless statues. Played straight in the finale with the Demon King Dal'Brad.
The D'denir statues in Warbreaker, which are really the Lifeless ultra-warriors known as Kalad's Phantoms.
The Neverending Story has a pair of winged statues that will fire upon you if you get too close to them.
Harry Potter: There is a guardian statue at the entrance to Dumbledore's study. Most of the time though, it only comes alive to ask for a password.
In the Simon Canderous books, the doorway to the arcology is guarded by a pair of living stone statues. But you can be given clearance so they let you pass.

Live Action Television

The Weeping Angels from Doctor Who.

Video Games

Blood has gargoyle statues that sometimes turn into live gargoyles. Much Paranoia Fuel ensues.
- Subverted in the expansion of the second game where every level and cutscene has at least one gargoyle statue hidden somewhere, but they're obvious props with low poly-count. It doesn't stop them from looking intimidating though, especially because most of the time they appear in places where they absolutely don't belong.
Dragon Age: Whenever you see a deactivated but otherwise intact golem, rest assured that it will start attacking as soon as you do something important.
- In addition, the Sentinels in the Mage starting quest could qualify as this.
Final Fantasy V has stone gargoyles guarding the four tablets.
Legacy of Kain: Lampshaded by a Genre Savvy Kain, who notes, "These statues were singularly inanimate. I knew better than to assume that they would remain as such."
Once the player reaches the castle portion of Resident Evil 4, they should be on the lookout for seemingly inanimate suits of armor.
The Legend of Zelda: Throughout the series, we have Armos and Iron Knuckles.
Thief: The villain of the third game starts animating stone statues in the late levels.
Warcraft: At one point in the third game there's a hallway with statues of armored men on either side. Further down the hallway are robotic golems which activate when you reach them (complete with "The statues are coming to life!" in case you missed the point). Not a very well-done one though, as the golems and the statues look nothing like each other.
The second Guardian (boss) and the Spriggan statue in La-Mulana. Also, wall reliefs will start shooting at you if you stand next to them for too long or strike them.
The room of the Hero Core boss 'Guardian' contains some statues of standard enemy machines you fought many times. Appearing to be decoration, they actually are idle machines and aid the boss once he lost a certain amount of Hit Points.
The first Chozo Statue in Super Metroid seems inanimate until you take the powerup it holds and try to leave, at which point the exit seals and it attacks you.

Western Animation

Dave the Barbarian. The particular model shown doesn't activate unless you touch it.