The actor appeared in the 1988 original film as Madmartigan alongside Warwick Davis, who played the titular sorcerer.
While Davis leads the new series, which was released earlier this week, Kilmer is nowhere to be seen.
Showrunner Jonathan Kasdan has now explained Kilmer’s absence, revealing that the actor decided to stay indoors during the pandemic.
“As Covid overtook the world, it became insurmountable,” Kasdan told Entertainment Weekly.
“We were prepping in the spring of the year that it was most happening. And Val reluctantly didn’t feel he could come out.”
Kilmer was diagnosed with cancer in 2015 and, after successfully undergoing chemotherapy and two tracheotomies, is free of the disease, but now speaks with the use of a voice box.
Kasdan recalled “going to see Val right after” the Willow reboot “started to get some momentum”, telling him: “Listen, we’re doing this. And the whole world wants Madmartigan back.”
Kilmer told the showrunner: “Not as much as I do.”
The actor’s involvement looked promising “until pretty late in the process, frankly”.
“He embraced me when I left,” Kasdan said. “He picked me up, and he said, ‘See? I’m still super strong.’ And I was like, ‘Great.’ We started building out the first season with the intent of having him appear.”
Although Kilmer is not in the first season, should there be a follow-up, he has made is clear that the actor is welcome any time he wants to return.
“We wanted to leave open the door to any possibility in the future and also honor the spirit of him,” he said. “We’ve tried to do that and work with him in a way so that he is felt and heard, if not seen.”
Kilmer was able to return for an emotional cameo in Top Gun: Maverick, which became one of the highest-grossing films of the year after its release in the summer.
Find The Independent’s reveiw of the Willow reboot, which is available to stream on Disney Plus, here.