role-play

perhaps they are too shy to play in public. you can divide them into groups and ask them to do the role-play in their own groups. it can not only help them to overcome their shyness but also enable more students to practise English. after group work you can ask volunteer group to play before the whole class. this time i think there will be more students because they have got themselves well prepared and what is more important is that they will think they volunteer not for themselves but for their groups.
besides you can use some ways to encourage them. for example, you can give volunteer students high marks. you always praise volunteer students even though they do not perform so well.

If you are teaching in the training course, then you have the authority to plan and require what efforts are required. The role-play excercise is not a game although it is a lot of fun. It is a teaching tool that helps students apply what is to be learned and the logic of the material. Do not make it voluntary just as you will not make the final exam voluntary. Role-play is an academic endeavor that commands the participation of the student to get an acceptable grade. Let the students know that their acting prowess is not as important as thier understanding of the information being presented to them. It is important to be able to express yourself and your knowledge in various ways. Go get'em.