“Say ‘Cheese!’”

Entry in progress—B.P.
 
Jokes on the saying include “I was never a photogenic person, mainly because when everyone said cheese I said ‘where?!‘“, “I wonder how many photographers have died after a big block of cheese fell on them, despite large groups of people trying to warn them,” “My friend is so vegan, he won’t even have his picture taken because he’d have to say cheese” and “When cheese gets its picture taken, what does it say?”
     
 
Wikipedia: Say cheese
Say “cheese” is an instruction used by photographers who are having difficulty getting their subject to smile. By saying “cheese”, most people form their mouths into what appears to be a smile-like shape. Additionally, the absurdity of saying “cheese” for no apparent reason can incite glee in some people.
 
As this practice became ingrained into modern society, it has taken on the simple role of a final warning before a photograph is taken. Often subjects will simply use the phrase “say cheese” as a cue to enter their final pose and to smile, neglecting to actually say “cheese”.
 
Over the years, many other words have been used in place of cheese. For comedic effect, a photographer might say “Say ______” filling the blank with a word relevant to the event or action he is photographing. (Ex. “Say wipe out” during a seaside photograph) Other times a person will fill the blank with an absurdist or seemingly entirely random word of their choice.
     
(Oxford English Dictionary) 
cheese, v
School slang. To smile.
1930 N. & Q. CLVIII. 119/1 Another slang use of the word ‘cheese’ was in vogue at Rugby School… This was with the meaning ‘smile’, both verb and noun.
1940 M. MARPLES Public School Slang 40 Cheese..to smile or grin (Oundle 1920+). 
 
cheese, n.
School slang. A smile. Also, esp. Photographers’ colloq., the word ‘cheese’ notionally or actually pronounced to form the lips into a smiling expression.
1930 [see CHEESE v.1 2].
1956 Punch 1 Feb. 179/1 They are almost certainly just saying cheese, cheese, cheese to hold their smiles while the news reel camera whirls.
1964 New Statesman 17 Apr. 612/3 Her deadpan face mouths a tiny ‘Cheese’ as she is pegged..into focus.
 
28 September 1942, Los Angeles (CA) Times, pg. A1:
When we got to the Sphinx the ack ack lieutenant unlimbered his camera and we got George to take our pictures in a pleasing pose. When he was ready, George shouted:
“Say cheese.”
“Cheese,” we all said. “Why?”
“That’s what you must say to smile for the picture,” said George.