Don’t Keep it Casual
The Western culture is much more informal than that of Eastern Asia. But there are some similarities here. In the West, we usually address doctors and teachers by their professional title and their last name. However, work stays pretty casual. Almost everybody addresses their superiors at work by their first names.
South Korea is a country where everyone strongly cares about showing respect towards one another, and avoiding addressing people by informal titles is one way of maintaining this behavior. Just like in the West, doctors must be addressed by their title and name, even in casual social situations.