If I'm remembering correctly, he implies that at least one of the eps was one in 1795 during Vicki's trail in which Nathan takes the stand against Vicki. Crothers makes a joke in the interview that one of the actors cross-examining Nathan couldn't remember his lines but Crothers knew his own lines as well as the actor's because he'd written them.
Now, I wonder who the other actor could have been? Crothers doesn't give any hints. But, well, just from watching DS ourselves I suspect we all have a very good idea.
I mean, one of the actors in question rarely if ever went up, while the other often rambled his lines and often accompanied that rambling with a gesture that I suppose he thought helped to convey every emotion possible, so what need did he have of words.