Se retourner vs. faire demi-tour

In the French language, different phrases can describe similar physical actions with subtle nuances. 'Se retourner' and 'faire demi-tour' both involve a change in direction, but they are used in slightly different contexts.

Se retourner

This phrase typically refers to quickly turning one’s head or body around to face the opposite direction, often with the implication of doing so to look at something.
En entendant son nom, il s'est retourné.
(Upon hearing his name, he turned around.)
Ils se retournent lorsqu'ils entendent un bruit étrange derrière eux.
(They turn around when they hear a strange noise behind them.)
Elle s'est retournée pour voir qui l'appelait.
(She turned around to see who was calling her.)

Faire demi-tour

'Faire demi-tour' means to fully reverse one's course or go back in the direction from which one came. It is more commonly used when referring to changing direction while walking or driving.
Le chemin était bloqué, j'ai faire demi-tour.
(The path was blocked, so I had to turn back.)
Quand il s'est rendu compte qu'il avait oublié son portefeuille, il a fait demi-tour.
(When he realized he had forgotten his wallet, he turned back.)
La route était trop glissante pour continuer; nous avons fait demi-tour.
(The road was too slippery to continue; we turned back.)

Summary

'Se retourner' involves a quick turn of the body or head and is often used reflexively in response to being called or wanting to look at something behind oneself. On the other hand, 'faire demi-tour' is more about reversing one's path completely, such as turning a vehicle around to head back in the opposite direction.