I know it's not always easy to do communicative activities with Japanese learners. My experience of Japanese culture is one that is very hierarchical which makes communication with superiors difficult. They also seem reticent about voicing their opinions in public. A teacher needs to be sensitive to these factors. Try to avoid using debates and activities that involve voicing opinions until you feel your students are ready for it.
Get what information you can about your students' jobs, backgrounds or any other factor concerned in status. Then use your knowledge of Japanese culture to group students. If you speak Japanese, or know somebody who does, find out whether certain set phrases are used in any communicative activity that you introduce. You can then give the students the English equivalent or allow them to use Japanese for these phrases. If you show that you're culturally sensitive, you've already removed one barrier to communication.
You can try the following two activities to get students speaking.
Write a dialogue on the board before class. Use comprehension questions to make sure the students understand it; this can also be done in writing. For real beginners, you can also check understanding with questions in Japanese. Split the group into two teams; the teams are person A and B in the dialogue. Practise the dialogue as a whole class, with one team repeating person A's statements after you and the other team repeating person B's statements. If you do this as a whole class, it gives the students a chance to practice the language in a safe environment. Nobody is listening to them or judging them.
Continue repetition of the dialogue until you think the students have mastered it. In the first lesson, you might choose to divide the class at this point and have them continue reading the dialogue in pairs.
For a more challenging activity, you can start removing some of the words from the dialogue, either during choral repetition or when they're working in pairs. Students continue reading but with fewer and fewer words until only a skeleton of the dialogue remains. You could even remove the entire dialogue.
In a future lesson when you feel the students are ready, you can elicit the dialogue from them or use one that they have written themselves.
WRITTEN SPONTANEOUS DIALOGUE
Put your students into pairs and give them a speaking activity to do. There is one large difference here: instead of speaking, students write. The first person writes down what they would like to say, the second responds in writing; and so on. When the dialogue is complete, the students can read it in pairs. My experience is that Japanese students focus on accuracy; so you could go around correcting (or helping students to auto-correct) dialogues before they start reading them aloud in pairs.
You can use these two activities on various occasions for different dialogues. Once students are familiar with them, they should gain more confidence and start to speak more freely. You can then start introducing other speaking activities. Also pitch any speaking activity at a level below what your students can produce in writing. If an activitiy is easy, there is more likelihood that your students will speak. Remember also to praise and encourage any effort your students make.