Smile!

Very nice.

Say cheese!

Nice.

That's different. Those guys are laughing.

The girl at left is laughing, the one in the background, not so much. Laughter is by definition spontaneous, no? It happens instantaneously. Laughter is prompted just as "say cheese" photos are, as the Talking Funny guys were, but it is without guile, it's uncontrollable. We hesitate a little to say the girl at right is laughing only because she is looking right at the camera. She could be mugging, posing. 

Same two girls. The dark-haired girl is camera-aware, she is posing, that is a "say cheese" smile. The lighter-haired girl may have been prompted by the brunette, the brunette may have photo-bombed into a posed photograph of the lighter-haired girl but that is a real laugh by the lighter-haired girl. 

Smiling and laughing. Both very nice. Laughing is better.