It is voluntary work, and you have to save money to go, as missions generally last up to four years. The experience tests both one's faith and one's ability to communicate with people, as it involves a lot of knocking on doors and asking total strangers if they would like to hear about the Church. Contact with one's family, friends and the outside world is limited. Transportation is usually by bicycle or on-foot. One must be determined, flexible, and able to take a lot of rejection.
Daily study is required to be able to remain focused and to answer any and all questions.
God's Army includes a scene where two non-members ask a pair of missionaries why black men could not hold priesthood positions in the Church prior to 1978, and why women cannot hold positions of authority. And missionaries are assigned to where they will be working. If they are to serve in Germany, Brazil, or Japan, a crash course is required to learn the language well enough to converse fluently. I have friends whose mission experiences were not so terrific, and friends whose experiences were so profoundly spiritual that they changed for the rest of their lives by them. It is also very funny in parts, and a unique film that has an ability to stick with you longer than many other films currently appearing in the theaters.