For me, memorization is the most difficult part of acting. With rehearsing, the dynamics of the scene and of the character you are portraying makes emoting come naturally. I have no problem crying actual tears, having become fully involved in my role, however the audience is usually too far from the stage to see tears, so actually crying is not necessary.
During a rehearsal, actors often “break up” and laugh when a deadpan or other emotional expression is required. But with sufficient practice one can usually avoid this happening during a performance. Effective acting requires the actor to be the character present in a “real” situation.
As long as actors are not conscious of the audience (key is never to look at the people in the seats), they will not “break character.” Sometimes actors will blank out and forget what they are supposed to say. Others actors, remaining in character, will be able to ad lib, cover the flub, and provide a link that helps the actor to get back on track. As long as there is no pause in the scene, the audience will never know.