You can quickly disable TouchID or FaceID to make sure that a Passcode is required to unlock your phone. This is useful when you're concerned that someone may force you to use your finger of face to unlock the phone.

Disabling TouchID or FaceID

On iPhone 8 or iPhone X:

• Press and hold the Power button and either Volume button

or

• Press the Power button 5 times in rapid succession

On iPhone 7 or earlier:

• Click power button 5 times in rapid succession

Either of these puts the phone in Emergency SOS Mode, where you can call for help, but more importantly it will need a passcode before unlocking.

• Cancel Emergency SOS Mode if you don't need it, clicking the Cancel button
  
• The phone will now require a Passcode to be unlocked

MORE INFORMATION

• The Cop Button (Engadget)
• Cops Can Force You to Unlock Your Phone (HuffPost)