Two keys to long talking head scenes: (1) You, as a writer, have to ABSOLUTELY justify the scene’s existence. You are POSITIVE the scene needs to be 4, 5, 6 pages long (or whatever). (2) What transpires in the scene not only has to move the plot forward… not only should reveal something about the characters… not only needs to mean something compelling to the script reader… it has to be entertaining as hell.
The actual content of the dialogue can be fascinating as with the U.S.S. Indianapolis monologue in Jaws. But you can also lean into the psychological interplay and subtext between the characters.
Perhaps the greatest example of this is a scene toward the end of the incredible movie Manon des sources. You can see it here. I don’t want to spoil it, but the Protagonist Cesar learns something from the old woman which completely upends his understanding of key events in the past. The clip is in French, but you don’t even need to understand the words. Watch Cesar’s facial expression (the brilliant actor Yves Montand).
In writing long talking head scenes, it also helps if you’re a helluva good dialogue writer as evidenced here in The Social Network (shout out to Aaron Sorkin.)