The leak that happens before the phone is even unlocked.
Every setting covered in our guide to WhatsApp privacy settings governs what happens only once the app itself is open. A lock screen preview happens well before any of that, a banner printing the sender's name and the first line of a message on a screen that has not been unlocked at all, readable by anyone simply standing near a phone left face up on a table.
It is the single most common way a private conversation becomes a public one by accident, since it requires no malicious intent from anyone nearby, only a glance at a phone that lit up at the wrong moment. A dinner table, a shared taxi, a phone charging within view of a colleague's desk, all count, and none of them involve anyone actually trying to pry.
On an iPhone.
Settings, Notifications, Show Previews offers three choices, Always, When Unlocked, or Never. When Unlocked is the useful middle ground, previews appear once the phone recognises your face or fingerprint and disappear the moment it locks again, so a glance at the phone in your own hand still shows the message while a glance from across a table does not. It is the setting most people settle on once they understand the choice properly.
The same screen allows a per-app override, so WhatsApp or Telegram can be set to Never while other, less sensitive apps keep their previews on Always, a sensible split for a phone that handles more than one kind of message.
On an Android phone.
The exact wording varies quite a bit by manufacturer, but the setting generally lives under Settings, Notifications, and usually offers a toggle for Sensitive Notifications or Show On Lock Screen. Many Android phones also let you choose, per app, whether a notification shows its full content, a redacted version, or nothing at all on the lock screen specifically.
Where a phone offers Notification Visibility as a per-app setting, apply it to the messaging apps you actually use for anything private rather than adjusting the phone wide default, which affects every app equally and often more than intended. Samsung phones tend to file the same choice under Lock Screen Notifications inside the notification settings for each app individually, while stock Android keeps it under Notifications, App Icon Badges and the app's own entry.
Notification history, a separate leak.
Beyond the lock screen itself, several Android phones keep a running Notification History or Notification Log, a scrollable list of everything that has recently arrived, including notifications you already dismissed from the lock screen. It usually lives under Settings, Notifications, Notification History, off by default on some phones and on by default on others, and it is worth checking rather than assuming.
A dismissed banner feels like it has gone, but if history logging is switched on, the same content can sit one tap away in that log for anyone who thinks to look, well after the original notification has cleared from the lock screen itself. Turning the feature off entirely, where the phone allows it, is the simplest fix once you know it exists.
Why per-app beats phone wide.
Turning off all lock screen previews solves the problem completely but at a real cost, every notification becomes a plain, uninformative banner, including the ones you would rather see at a glance, a calendar reminder, a ride confirmation, a delivery update. The per-app route keeps the phone genuinely useful while closing the gap only where it actually matters, which is worth the extra minute of setup.
Testing it properly.
The only reliable check is a real one. Lock the phone, ask a friend to message you, or send yourself a message from a second device, and look at what appears. A setting that seemed correctly applied sometimes leaves one app untouched, particularly after switching phones or restoring from a backup, and thirty seconds of testing catches that before it matters. Repeat the test after any operating system update, since these settings occasionally reset without warning when a phone updates itself overnight, and again after installing a fresh copy of a messaging app rather than updating an existing one.
Where this matters most.
A confirmation for a dinner date arriving while the phone sits face up on a conference table in the Zuidas is exactly the scenario this setting exists for, and it applies equally whether the companion confirmed is filed under black hair or the 26 to 29 band, since the setting protects the notification itself, not the content of any one particular thread. Whatever the rates page eventually quotes, none of it needs to appear on a screen nobody meant to show it.