"What gets tested, gets taught" is an American teaching proverb.
Unfortunately, many standardized ESL exams - at least in the United States - focus more on passive skills like listening, reading comprehension, vocabulary, and grammar than speaking or writing. Testing conversation skills takes more time, money, and resources. As a result, many ESL courses and programs spend far too little time teaching conversation skills.
Another factor limiting the number of conversation classes, in the public education programs like adult education and community colleges, is many potential students prefer to work as soon as possible. Programs, therefore, focus more on lower levels where speaking skills remain too weak for more in-depth conversations.
As somebody who has taught several conversation classes and many more English classes in community colleges and adult schools, I have found that systematically planning lessons is very helpful. You can't just pick a random topic of personal interest. Know your students and pander to their interests. If you have a film student or musician, create a lesson that appeals to them. Some topics, however, hold wider interests.
You also need to create a wide range of questions, scaling from the simple and direct to more complex and philosophical. You also want to anticipate vocabulary needed - and probable "good mistakes" in terms of words easily confused.
Several popular books combine short lists and a few vocabulary words. Heads Up and Conversation Book 1 and 2 - being the most popular. When I taught an Advanced Conversation class at Santa Monica Community College, students didn't want to spend money on books with a handful of questions and vocabulary words.
As a result, I ended up creating worksheets with 30 questions and 10 vocabulary for the class that meet two hours at a time, twice a week. I found that adding several proverbs and quotations, from a variety of perspectives, to introduce a topic - and some vocabulary - made students more comfortable, added a more academic quality to the lessons, and introduced new vocabulary. Conversations also tended to be more interesting, engaging, and memorable when students had a chance to reflect a bit before working in pairs or groups.
I compiled those conversation lessons, on 45 different topics, into a book called Compelling Conversations:Questions and Quotations on Timeless Topics a few years later while between teaching gigs. You can find sample chapters at Home
You will, I suspect, find that you need some set format to keep the class moving. While other instructors might be able to just wing it, I have always found advance preparation - even over preparation - very helpful. This approach is especially important when the class content seems - superficially - very simple.
Balancing accuracy and fluency will remain an issue. I tilted toward fluency in the discussions, noted student errors, and shared "good mistakes" with the entire class after a group discussion of the topic. This indirect method of correction works far better with adult students, particularly professionals and seniors who may have "dignity" issues.
If I had the pleasure of teaching a strictly conversation class again, I would add videotaping to the class routine. You might, for instance, conduct mock interviews and post them on a class website for self-evaluation and peer-evaluation. Students are often learn a great deal about their communication styles and English skills from watching themselves on video.
Teaching conversation can be a very satisfying experience. Enjoy!