Jonathan Harris did not join the cast of Lost in Space until after the show was already in production, not being in the pilot. At this point the billing had already been finalized, so through contract negotiations, he was able to be billed as a 'Special Guest Star' for every episode. According to him, starting the entire "Guest Star" billing in television period.

As from the Interview Transcribed here.

I called Allen and said, "I solved your billing problem."

 

"Eh? What?," said Mr. Charm.

 

I said, "I will accept last position: Special Guest Star Jonathan Harris."

Or if you care to listen, from the horses mouth. Here is a direct mp3.

Jonathan Harris did not join the cast of Lost in Space until after the show was already in production, not being in the pilot. At this point the billing had already been finalized, so through contract negotiations, he was able to be billed as a 'Special Guest Star' for every episode. According to him, starting the entire "Guest Star" billing in television period.

As from the Interview Transcribed here.

I called Allen and said, "I solved your billing problem."

"Eh? What?," said Mr. Charm.

I said, "I will accept last position: Special Guest Star Jonathan Harris."

Or if you care to listen, from the horses mouth. Here is a direct mp3.