At 91 years old, actor William Shatner, who will turn 92 later in March, says that his approaching death motivated him to do the documentary "You Can Call Me Bill."

When he was 90, Shatner, who is widely known for his role as the character Captain James Kirk on "Star Trek," blasted off on a brief, real-life jaunt into space aboard a Blue Origin rocket, becoming the oldest person ever in space.

And at 91, he's apparently still able to ride horses. "After this interview I'm gonna get on a horse," he told Deadline. "I'm not going to get in a wheelchair. I'm gonna go on a reining horse and practice."

But Shatner recognizes his own mortality.

"I've turned down a lot of offers to do documentaries before. But I don't have long to live. Whether I keel over as I'm speaking to you or 10 years from now, my time is limited, so that's very much a factor," he told Variety when asked why he chose to do the documentary. "I've got grandchildren. This documentary is a way of reaching out after I die."

"In the movie, I didn't just want to go on about I did this or that when I was 7 or this is my favorite color. I'm trying to discover something I've never said before or to find a way to say something I've said before in a different way, so I can explore that truth further," he said, according to Variety.

"I read all the time — newspapers and books. I'm feeding my mind. The sad thing is that the older a person gets, the wiser they become and then they die with all that knowledge. And it's gone. It's not like I'm going to take my ideas or my clothing with me. Today, there's a person going through some of my clothes in order to donate or sell them, because what am I going to do with all these suits that I've got? What am I going to do with all these thoughts? What am I going to do with 90 years of observations? The moths of extinction will eat my brain as they will my clothing, and it will all disappear," Shatner said.

He claimed that "what does live on are good deeds. If you do a good deed, it reverberates to the end of time. It's the butterfly effect thing. That's why I have done this film."

The documentary directed by Alexandre O. Philippe is slated to premiere at SXSW.

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