Characters
You're not making a joke...
— Tony · West Side Story
Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker seems to realise that the other person is being serious rather than joking. The trailing ellipsis suggests a pause, perhaps disbelief or dawning acceptance, as if the earlier remark first sounded like a gag. It reads as a tentative check on how seriously to take what was said.
Context reviewed