It's not like that, depending on the church sponsoring the trip. I did missionary work in East Africa, including Djibouti: Hence the name... We went over, showed them what we believed in, experienced their culture, took surveys of locals regarding morality, culture and religion, and slept in youth Hostels. It was an overwhelming experience that has shaped my life in so many different ways, and never once did we force-convert anyone. It has been, up to this point, my defining moment, and an experience that will have endless impact on my life. If you are considring it, and trust the church you are going with to me non-judgment oriented and focused on education not indoctrination, then GO GO GO!!! It will be worth it.
Hope it works out for you.
gabe