Peter Hurley is a very well known head shot photographer in New York and this video details one of his well known secrets of getting a genuine smile from his subjects.
Hurley, Peter (2013). It’s All About the Squinch [user-generated content online] Creat. Peter Hurley. 19 November, 2013. 15 min 33 sec. Available at: https://www.youtube.com/watch?v=ff7nltdBCHs (Accessed 31 July, 2016)