While not regarding emotions directly, Littlewort et al. studied the discrimination of facial expressions of spontaneous and posed pain. However, I don't remember the number of subjects they tested, as their main aim was an automated system to accomplish that task.

Conference Paper Faces of pain: automated measurement of spontaneousallfacial...

This is very interesting, I sure am gonna read this. Thank you very much!

There are some differences between spontaneous and deliberate facial expressions.

Spontaneous facial expression are shorter in total duration but have abrupt and/or stepped offset and onset and the intensity of the expression is not constant (the expression will fade then increase and then again fade in exemple).

The deliberate expression are more assymetric (but this studie,  http://www.pitt.edu/~kschmidt/schmidt06DS.pdf , has not replicated the finding concerning the assymetry of smiling)

Sometimes, somes facial action present in spontaneous expression will be absent in the deliberate one, or a suplementary facial action will appear in deliberate expression.

I hope that this will help you and that my english level will not impede your understanding of my answer. I have tried to organize  bit the reference that I know on the subject below

Différences in timing :

http://www.pitt.edu/~kschmidt/schmidt06DS.pdf

https://www.paulekman.com/wp-content/uploads/2013/07/Not-All-Smiles-Are-Created-Equal-The-Differences-Bet.pdf

http://www.pitt.edu/~jeffcohn/biblio/cohn&schmidt2004.pdf

https://www.paulekman.com/wp-content/uploads/2013/07/Felt-False-And-Miserable-Smiles.pdf

- More irregularities in the timing of the muscular contraction (stepped onset and offset in exemple) :

https://link.springer.com/article/10.1007/BF00990794

http://onlinelibrary.wiley.com/doi/10.1002/ejsp.2420200502/abstract

https://www.researchgate.net/publication/227146309_Anatomically_based_measurement_of_facial_expressions_in_simulated_versus_hypnotically_induced_affect

- An offset more rapid in deliberate facial expression :

http://link.springer.com/article/10.1023%2FA%3A1024952730333

Differences of structure :

- Deliberate emotional expressions are more assymetric

http://onlinelibrary.wiley.com/doi/10.1111/j.2044-8309.1991.tb00929.x/abstract

https://www.paulekman.com/wp-content/uploads/2013/07/The-Symmerty-Of-Emotional-And-Deliberate-Facial-Actions.pdf

https://www.paulekman.com/wp-content/uploads/2013/07/The-Asymmetry-Of-Facial-Actions-Is-Inconsistent-With-Models-.pdf

- Presence or absence of facial action in posed facial expression that is not seen in spontaneous facial expression :

https://www.paulekman.com/wp-content/uploads/2013/07/Smiles-When-Lying.pdf

Article Anatomically based measurement of facial expressions in simu...

Thank you very much Julien, this is greatly appreciated. Very detailed answer and very informative. This gives me a lot of good info and I can't wait to start reading! (By the way your english is excellent!)

Merci beaucoup!

SUM IT UP FOR FUN BUT EDUCATIONAL FOR SURE

Watch "Lie To ME" on Netflix, 

Microfacial Analysis which not only gives hard facts, but substantiates it with hard core current faces and facts we can see. THis is a must show.  Please let me know what you think,

Michelle

Michelle,

Lie to Me is a good show for which Paul Ekman was an adviser. Pretty much of the fact on facial expression and what we can know and what we can't know with them are supported by scientific proof. But on the deception detection topic, the focus on facial expression, and more specifically on micro-expression is exagerated. A better focus on interviewing technique and strategies and on manipulation of the cognitive load by these techniques and strategies would have give a better picture of the possibilities given by the research on lie detection. 

But a very good show, I'm a fan ^^

Julien