The obsession with producing tears as a barometer for "good acting" obscures the fact that it is just as, if not more, difficult to laugh genuinely. From a UCLA study on laughter:
For the study, Bryant recorded the spontaneous conversations of college roommates. From these recordings, he collected 18 spontaneous laughs, which he considered to be genuine. He then enlisted a different group of co-eds to laugh on command. From this exercise, he recorded 18 fake laughs of the same length as the real ones....In the first round, the participants were asked to determine whether the laughs were real or fake, and the students could usually tell the difference.
Apparently fake laughter and real laughter are controlled by two different muscle groups, making the reproduction of genuine laughter difficult, mostly having to do with how quickly the windpipe can open. Ignore the following if science is not your thing.
With real laughs, the proportion of breathy parts in the call was consistently greater than with fake ones. Bryant attributes that to the particularities of the emotional vocal system. The emotional vocal system has more efficient control over the opening and closing of the windpipe, thus allowing people to emit air rapidly during genuine laughs. In fact, during genuine laughs, the windpipe can open and shut at a rate that approaches the apparatus's maximum potential...
In contrast, the speech system, which is responsible for fake laughs, controls the dynamics of the vocal tract differently and can't open and close the windpipe as quickly...
Humans, Bryant says, have evolved a particular sensitivity to fake laughs because the stakes of making a mistake can be very high. Researchers have found that genuine laughter releases the soothing hormone oxytocin, which promotes a feeling of affiliation and cooperation. False laughter that succeeds in passing as the real thing can therefore be used to take advantage of another person.
"You have to be vigilant, because you want to discern whether people are trying to manipulate you against your best interests or whether they have authentic cooperative intentions."