Stick'Em Up

Ever notice how in hostage situations, someone just has to jam the gun into the person's back for them to know you mean business? The victim can't see the weapon, so sometimes a resourceful, desperate, and usually good person takes advantage of that by using some random stick or anything that remotely resembles the shape of a barrel to hold someone at "gunpoint".

Sometimes the deception extends beyond the victim to the audience as well. Until The Reveal, anyway.

It can work with knives too, but guns seem to be the preferred choice when an option. Sub-Trope of Brandishment Bluff.

Examples of Stick'Em Up include:

Anime and Manga

An early Trigun episode plays the "finger" version straight.
Kirihara in Darker than Black, while shopping, stuck her battle lipstick into the back of someone resembling BK-201. Who was BK-201 in civil identity... that is an awkward lad she already knew, so he only had to explain buying a bra and a wig without mentioning he hides someone from Yakuza.
Used in Suika, aka Wet Summer Days. Kamishiro Souji is attacked by some local toughs who tell him of their intention to have their way with his girlfriend, the daughter of a local artist (who is believed by some to have killed his wife). Cue the girl's father coming up from behind and pressing a bit of metal into one of their backs, telling them it's his pallet knife, and then describing how his knowledge of anatomy as an artist gives him the knowledge to be a most efficient killer.

Comic Books

This happens in The Adventures of Tintin at least twice.

Film

Bandits did this with a permanent marker, a bank security guard and Bruce Willis. Great movie.
In Batman Begins, Batman used a stapler for the barrel and the clicking noise to simulate a gun cock.
Parodied in Field of Dreams, with Ray trying to "kidnap" Terence Mann by faking a gun in his pocket. Terence responded with a crowbar.
Done in Out of Sight, also with a highlighter or something.
In Kiss Kiss Bang Bang, when a malcontent forces Perry along with a gun to his back, Perry calmly explains that "Professionals prefer to have about three feet distance between the gun and the guy. Otherwise, he could just do this." He then grabs the gun out of the guy's hand and turns it on him.
The Chase from the movie of the same name starts when Charlie Sheen takes an heiress hostage with a candy bar.
Referenced in Ratatouille: one version of what sous chef Horst did to end up in jail is that he "once robbed the second biggest bank in France using only a ball-point pen."
Done in pant-wetting fashion in True Lies with a tube of lipstick under the chin.
John Dillinger uses an improvised wooden prop gun to escape prison in Public Enemies. Truth in Television too.

Literature

Done by Anathema Device in Good Omens to get into an American air base. Used with a bit of Exact Words, as she told the soldier to put down his gun or else she will regret what she'll have to do. Her internal monologue continues with, "Because if you don't put down the gun, you'll find out this is a stick and then I will regret having to be shot."

Live Action TV

Surely done on 24.
MacGyver's done this with a broomstick at least once.
In one episode of the original Superman TV series, Clark Kent did this with his finger.

Clark It's just a finger
Baddie You got a finger made of steel, buddy

The First Doctor does this in Doctor Who with a branch. "The Time Meddler", aka the Meddling Monk, finds out... and the Doctor threatens to smack him upside the head if he doesn't behave.
- Didn't the Fourth Doctor do something similar with his "deadly jelly babies"? (Not a stick-up, of course, but I seem to recall them being passed off as some sort of weapon.)
  - Poison. He claimed they were poisonous, and threatened to force-feed them to his hostage.
In an episode of the Scottish murder mystery series Taggart, a police detective used a piece of pipe to get an IRA killer to lower his gun. When he realised he's been fooled the IRA man couldn't help laughing.
The Adventures of Lano and Woodley used fingers in the episode Game Show God.
White Collar: Neal threatens to shoot a man by jamming a birdhouse against his back.
Omar in The Wire does this in season 5. But being Omar, you would expect him to have a real gun.

Radio

A French radio show had a baddie holding up twins using a real gun and a pipe, after pointing out he was ambidextrous.

Video Games

In the original Castle Wolfenstein, the player can surprise guards and hold them at gunpoint, even with an empty gun.

Web Comics

Antihero for Hire: Dechs uses his finger at one point; this is seen as a major development, since Dechs -always- uses his gun.
In No Rest for The Wicked, a flashback shows Clare holding up a man by pointing her metal finger into his back and calling it a knife. When her baby wailed, it really ruined the effect.

Western Animation

Parodied in Buckaroo Bugs and many other Looney Tunes cartoons.

Web Original

In Sarah's debut episode of Lonelygirl15, "Crazy Emo Chick", she introduces herself to Daniel by putting lipstick to his back, announcing that it's a gun, and demanding that he puts his hands in the air, and that Bree and Jonas turn off the camera. It turns out she's just playing a practical joke.

Real Life

Truth in Television. Just don't press your luck either way.
Averted in Britain where guns are incredibly hard to get hold of; you need more permits and licenses than you do in the U.S.A. Most people who have guns in Britain have them for hunting animals or as collectibles. Because of this, gun crime (and fake gun crime) is very low although knife crime is unfortunately high.