Putting a Hand Over His Mouth

Similar to the Finger Muzzle, this is a more direct and easier way of shutting someone up. If Bob is being kidnapped, the best way to prevent him from crying out is by grabbing him and covering his mouth. Not only that, but Alice could easily do it if she just wanted him to stop talking for some reason.

This gesture can be both threatening or playful, depending on the context. Rescuers may sometimes use this if they have to sneak up on a friend from behind and don't want them alerting nearby foes by yelling in surprise.

Compare with Finger Muzzle, Bound and Gagged, "Shut Up" Kiss, Standard Female Grab Area, and Finger on Lips.

Examples of Putting a Hand Over His Mouth include:

Anime and Manga

Jessie does this to James several times in the Pokémon episode "Holy Matrimony!"
Played for laughs during the Griffith rescue in Berserk, where Princess Charlotte has a quite uncharacteristic temper tantrum at the thought of not going with the now-exiled Band of the Hawk in order to stay with Griffith. To shut her trap, Casca, Guts, Judeau, and her maid Anna all put their hands over Charlotte's mouth.
In Fruits Basket when Kagura almost lets slip to Tohru about Kyo's true form Kyo covers her mouth with his hand.

Comic Books

Happened to Lois Lane quite a bit in the Golden Age Superman comics.
Batman will occasionally surprise his enemies from behind this way.

Film

In Disney's version of Peter Pan, Wendy is silenced this way twice, once by Peter and once by a pirate.
Pocahontas hushes Nakoma this way when John Smith approaches.
Happens to Littlefoot in the first Land Before Time sequel when he is captured.
Flora shushes the other fairies this way in Sleeping Beauty. A goon also does this to Phillip for a second.
A monkey does this to Mowgli in The Jungle Book.
In The Great Mouse Detective, Basil does this to Dawson, as does Ratigan to Olivia—before she bites it.
Mulan briefly silences Mushu this way during his "dishonor" rant.
In The Hunchback of Notre Dame, Victor does this to Hugo when he tries to spit on a mime.
In Tarzan, a villain gags Jane this way, causing her to bite him.
Lilo and Stitch: Nani does this to Lilo near the beginning, and shortly after wraps her entire arm around her mouth as well. Lilo licks her.
In The Nightmare Before Christmas, Sally's disembodied hand does this to Santa Claus when she's rescuing him.
Atlantis: The Lost Empire: "I have some questions for you and you are not leaving this city until they are answered."

Literature

In the first book of The Wheel of Time series, Rand's father uses the friendly variation of this to keep him from crying out when they're being hunted by trollocs. Rand nearly breaks his arm in his panic to get free.

Live-Action TV

Happens a few times on iCarly. Most notably, Carly licks Sam's hand at one point, exclaiming that it tastes like peanuts and dirt.

Western Animation

Often happens to April and Irma on Teenage Mutant Ninja Turtles.
Happens frequently on Inspector Gadget to Penny.
The spies on Totally Spies! frequently do this to each other.
Four instances in Liberty's Kids; James does it twice to Sarah to prevent her warning any nearby enemies; Sarah gives one to Henri at one point and one to James to stop him mocking the buying of St. Lawrence