In the original pilot (*The Cage*), we see that Spock smiles:


![enter image description here][1]

As pointed out in the accepted answer to [this question][2], 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?*  


  [1]: https://i.sstatic.net/K3Bwn.jpg
  [2]: https://scifi.stackexchange.com/questions/74123/what-is-the-in-universe-reason-for-spock-smiling-in-the-menagerie