There are a lot of effective techniques out there to build on your usual dialogues. The other students aren't listening because, having heard and/or practiced the pre-written conversation, there aren't any more challenges. The key, then, is to build additional challenges into the activity.
Here are some ideas.
Different dialogues: If everyone in the class has a different dialogue, then the performers can ask 2-3 comprehension questions at the end. Award points to pairs that answer quickly and correctly. I'm assuming that your class doesn't have more than a dozen people, as you'll waste a lot of precious class time doing this with a large class. For big classes, break everyone into groups, and let them run the activity themselves.
Same dialogues: Have the students perform the same dialogue with different partners. Read once, switch roles, and then switch partners. As an extension activity, switch partners once more, and have the students continue the dialogue (building something along the lines of a role play).
Or you could work on a different focus for each reading. As above, students switch partners each time the dialogue is read. On the first run through, each student holds an imaginary microphone as if in an interview. This encourages eye contact. Next, in order to reduce distance, the pairs should place a hand on one another's shoulder. On the third reading, students work on using body language in the dialogue. Lastly, on the fourth run through, students stand back to back, so as to practice speaking clearly and at an appropriate volume. Of course, they should then finish by putting everything together, reading the dialogue without referring to the script (if possible).
These are only a few ideas, and all can be mixed and matched to extend ta dialogue activity. Challenge your students, and they'll stay interested.
Chris Cotter
www.headsupenglish.com