I don't know how it works elsewhere in the world, but for virtually every British child, the nativity play is the standard introduction to live performance. Every nativity play contains a microcosm of every play ever produced. All illustrations by Giotto.
There will have been ferocious competition for the female lead. Reactions to the ultimate choice will vary from shoulder-shrugging "meh, it's not a very good part anyway" to "I know it should have been me, but I was just too good to play opposite Joseph". These opinions will be expressed in audible whispers at every rehearsal. If Mary is ever absent, there will be a flurry of eager readers-in, eyeing the director and hoping that the miracle will happen and someone will say "we've made a terrible mistake, you are clearly much, much better and shall replace her right now".
It will have been very easy to cast Joseph as only two boys will have been eligble to play the role at all and one of them will have been absent on audition day. The performer will take one of two forms. He will either be sublimely confident or a gibbering inaudible wreck. In either case, he will be mollified by being allowed to wear a beard. Joseph and Mary will behave as though they are plague-ridden and refuse to hold hands. Both will repeatedly say "we'll do it on the night".
These three will be clowns. They will probably be the most experienced performers and know that the best parts do not necessarily have the most lines. They will make the whole cast laugh and everyone will adore them. The director will not adore them. At least one of them will have already played the role before and will know how to do it better. They will not learn their lines as they will assume that the other two will. During the performances, they will suffer collective memory loss and go through their scene three times in different iterations. On leaving the stage they will have a massive and highly audible argument about whose fault it was.
Gabriel will either be a reject Mary (see above), or taking their first important part. In either case the role will be taken extremely seriously and the performer will shoot resentful glances at unruly cast members. The director will learn to dread those faintly pursed lips and the whispered "can I just ask you something?"
Angels are obsessed with their costumes. Furtive arguments will break out over length of dress and amount of tinsel. They will always have a solo spot of some kind which will be impossible to rehearse as the whole team will only be available at the dress rehearsal.
This is the ultimate home for the too young, too shy and too grumpy. They will want to be part of the play, but will feel that the whole project is an excuse to chat to their mates. Some will find relative anonimity a relief, but there will always be a contingent hunting for ways to make themselves noticeable. In a year's time, such performers will be playing Magi (see above). Shepherds become obsessed with props. "Can I have a stuffed sheep?" If there is a Head Shepherd, he will be the other potential Joseph (see above). He will need a special prop and woe betide anyone who tries to move or use it. "That's my crook!" followed by a solid thump and tears is standard at this point.
If you think the above is cynical, all I can say is visit your local Operatic or Dramatic Society and look for yourself. The person waving their arms, looking despairing and trying not to have a breakdown is the director.