The floating heads dialogue problem occurs when two (or more) characters are talking and the author forgets to fill the scene by involving the setting and showing how the characters move during the setting.
Closely related is the ‘sit down and talk’ scene which sneaks its way into a draft quite easily. Though the dialogue may be compelling, the scene itself is not dynamic enough to be engaging.
Whenever possible, have the characters move around and doing things while they are talking. Preferably, have them interact with the setting. This is a great way to keep the scene dynamic and engaging but also allows you to sneak setting descriptions into the narrative without disturbing the immersion.