When a character is silenced, prevented from speaking, or interrupted during dialogue.

This applies in situations such as when another character places a hand over their mouth, a sudden noise or action prevents the character from speaking, when a character is suddenly gagged, etc.
While sudden interruption is common situation where this tag applies, this tag can apply even if action of silencing another is not sudden if intention to silence is present.

If character is already prevented from speaking, consider using gagged or covering_mouth if applicable.