An actress friend is concerned because she can't make herself cry. Here's my take on stage crying:
1) Very few people can turn on the waterworks at will. This is just a truth, and all actors need to admit it to themselves. In play X, if asked to shed tears, an actor may not be able to do it. He needs to admit that he may not be able to do it and come up with some other plan. (This actor is probably a really really good actor. Sometimes good actors can't shed tear; sometimes bad actors can shed tears. Shedding tears is an auxiliary ability, like stage combat. It doesn't define good acting.)
2) "Some other plan" is faking it. If shedding tears isn't an option and the play calls for crying, you need to fake-cry. There's NO shame to doing this. Acting ISN'T about feeling real feelings; it's about convincing the audience that you're feeling real feelings. All that method stuff -- the stuff that helps you feel real feelings on stage -- is useful, because the easiest way to convince someone that you're having real feelings it to actually have real feelings. But it's important to remember that "having real feelings" is not the goal-- it's one tool to help you reach the goal. And when that tool fails, you should toss it and choose another.
An actors might refuse to fake-cry because...
1) He thinks the goal of acting is to feel real feelings. But he's wrong. The goal is TO TELL A STORY -- to convince the audience that he is feeling real feelings.
2) He doesn't enjoy acting unless he is feeling real feelings. I sympathize, but he's being selfish. Acting isn't about the actor serving his own needs; it's about the actor serving the play. The play doesn't care whether or not the actor really feels anything. If the play cares about anything, it cares about getting its story gets told.
3) He's scared that if he fake-cries, he'll do it badly and it will be obvious that he's faking. Okay, so get better at it! Rent movies of with people crying in them and study their faces. Study their body movements, etc. This sounds silly, but I strongly believe that ALL acting schools should have a class called "Faking It." In that class, students should learn how to fake crying, laughter, orgasm, etc. All actors should feel secure that if they can't muster up the real thing, they can effectively fake it. They should never feel that they can neither really do it nor fake it. If they feel that, they'll panic. (And I bet an actor who is really secure with his fake-cry will actually be able to shed real tears more often than an insecure actor. One of many factors that keeps the insecure actor from crying is fear of failure. It's like trying to get an erection when you're worried about premature ejaculation.)
What about tears? Well, there are all sorts of tricks you can play -- and they're EFFECTIVE. One is to play the action of holding-back-tears. (When real tears roll down an actors cheeks, it's impressive in a pyrotechnic sort of way, but it doesn't actually serve most scenes. It's too much of a release: too cathartic. It kills all the tension. Holding back is usually better. For a rare counter-example, see Emma Thompson in "Sense and Sensibility." And think about how rare such scenes are.) Wiping away non-existent tears works really well too. Don't wipe them from your cheeks -- wipe them from you eyes. The audience can tell there are no actual tears on your cheeks (in a close space), but they can't tell that there are no actual tears in your eyes.
(When I was playing Uncle Vanya, I came up with a great trick (it helped that I was directing): I told the actress playing Sonya to come up to me and wipe away my (fake) tears. This is what magicians call "misdirection.")
Fake it. Sell it. And have fun selling it. When they praise you on your realism, inwardly smirk and think, "SUCKERS!" They're not really suckers; they're theatregoers. Theatregoers WANT to believe. They are paying to be conned. Con them, goddammit!
As for the movies: bah! Hardly anyone really cries in movies. It's either glycerin or tears that finally come after a zillion takes -- and you're only seeing take One Zillion. If you want to ruin a bunch of movies for yourself, watch scenes in which people cry and notice the inevitable cut right BEFORE the appearance of tears. BAD GUY: You must pay the rent. SWEET POLLY: But.. but.. *sniff*... [Cut back to BAD GUY] BAD GUY: Boo ha ha ha ha HAAAA! [Cut back to POLLY with tears running down her cheeks.]
It's editing and special effects.
Some screen actors who CAN cry on cue opt for glycerin instead -- because it looks better than real tears.