The ninth season finale, which aired on Tuesday night, climaxed with Ducky (David McCallum) suffering a heart attack.
"David and I have been talking about it for months, and he's been preparing for it," Glasberg told Entertainment Weekly.
"He was absolutely 100% willing to play along, and we wanted to be very secretive about it and keep it under wraps, and it's really going to give his character a new perspective on life coming back from what he experienced."
McCallum signed a new NCIS contract earlier this month, with Glasberg confirming that Ducky would return in season ten.
"I'm not going to play games with people," said the showrunner. "People know he signed a new contract and he is coming back to see us, so it'll be more about the way the character evolves [after the heart attack].
"And as a medical examiner who deals with the dead, and deals with death on a daily basis, to have an experience like that is significant. And we'll play that throughout the season."
NCIS was picked up for a tenth season in March, as part of an 18-show renewal by CBS.
The series returns to Tuesdays at 8/7c in the fall, airing on Fridays at 9pm on FX in the UK.