In the original pilot (The Cage), we see that Spock smiles:
As pointed out in the accepted answer to this questionthis question, the reason provided was that Spock was 'grinning with relief'.
It got me to thinking though; evidently at that stage of planning Spock wasn't entirely restricting his emotions; he would still have emotional displays at time.
My question then is this: out of universe, when and why did the character of Spock restrict his emotions?