A few examples:
He gulped from his glass(,) then stroke and twisted it.
He blinked at her for a few second(,) then smiled.
She stared at his chest for a moment(,) then agreed with a nod.
As you can see, all of these sentences have a dependent clauses after "then" (they are not complete sentences). On the other hand, they sound like nonrestrictive clauses (they are not necessary for the first sentence).
So I'm confused ... should I add a comma before "then" or not?