Imagine your dream grandpa: He's the best storyteller, dotes on your grandma, has plenty of sage wisdom to share and calls you "cutie."

Oh, and he's also Big Bird.

That's basically Caroll Spinney, the 80-year-old puppeteer who has played the 8-foot-tall feathery friend, as well as Oscar the Grouch, on "Sesame Street" for the past 45 years. Yep: The man who gives voice to some of toddlers' most beloved characters is an octogenarian -- a fact that remained something of a secret until Spinney agreed to participate in a documentary about his life.

Spinney wrote a memoir about a decade ago, though he says it's available only on eBay nowadays. The film "I Am Big Bird: The Caroll Spinney Story," which premiered at the Los Angeles Film Festival in June, aims to bring his story to a wider audience than the book. The movie shows Spinney performing on the set of the PBS program and also features archival footage to spotlight some memorable moments from his career -- including his relationship with Jim Henson, traveling abroad with Bob Hope and filming the 1983 special "Big Bird in China."

When directors Dave LaMattina and Chad Walker set out to make the film five years ago, they assumed there was a good chance they'd be capturing Spinney during his final year on "Sesame Street." "It quickly became evident," LaMattina says with a laugh, "that that was just not going to happen."

"Nah," says Spinney, who is sitting beside the filmmaker in a Los Angeles hotel. "If you rest, you rust."

Oscar the Grouch -- who, Spinney explains, was first made out of a ratty bath mat -- is strewn on a nearby chair. Spinney's reluctance to retire is at the core of "I Am Big Bird," as the film explores the personality traits he shares with the characters he portrays.

Even at 80, the physically fit Spinney keeps the long coif he sported back in the 1970s. He's sharp and energetic, though he does have a tendency to let his conversations ramble.

It's clear that Spinney views Big Bird as an extension of himself. In one of the film's more poignant scenes, he recalls an instance in which the costume was vandalized by a group of teens. Lying disheveled in a pile of dirt, it was missing a big patch of feathers that had been ripped off the bird's chest, and an eye was hanging by a few threads.

"It was like seeing my little boy wrecked," Spinney says, his voice heavy with emotion. "I didn't have a great time at school. I was teased because of my name, my size, my big ears -- and shame on them. I was a good kid who just wanted everybody to like me. That's the way Big Bird is. I'm reliving a better childhood through that bird, who is a child, and I think I'm awfully lucky to have that child."

Still, playing Big Bird is demanding. After he pulls on the character's orange legs like a pair of pants, the bird's yellow body is placed over his torso. To control Big Bird's head, he uses his left arm, which he has to hold in the air constantly. He can't see through the costume, so he interacts with the other puppets by watching a monitor that is strapped to his chest via what he refers to as an "electronic bra." Oh, and the lines: Since he's hidden from view, he doesn't have to memorize them. Instead, he cuts up his script and tapes it to his monitor. It's a lot to juggle.

But Spinney has transcended the mechanics of the puppeteering and finds something more fulfilling in the role. "I've always said that, of all the characters, Big Bird is more human than anyone on the show -- even the humans. He has human foibles and emotions," the actor says.

"I Am Big Bird" comes in the wake of the 2011 documentary "Being Elmo," which focused on puppeteer Kevin Clash, Spinney's longtime colleague. The film was a critical success, but Clash later became the subject of unsavory headlines when young men claimed he had engaged in underage sex with them. He resigned from "Sesame Street," and the lawsuits have been dismissed.

The new documentary raises some interesting questions about why Elmo eventually usurped Big Bird's rank as the program's most popular puppet. Over the years, the film posits, the show slowly adjusted its aim to a younger audience. Because Elmo is 3 and Big Bird is 6, the red furry creature increasingly proved more relatable to viewers.

"It was a little disappointing," Spinney says. "So I had to get used to that a little bit. But it was not a bad decision on their part -- and it was nice to know they didn't say, 'Hit the road.'"

In the '80s, though, Big Bird was so cherished by the children of America that NASA asked Spinney if he'd be willing to orbit the Earth on a space shuttle to help generate interest in space exploration among youngsters. Despite the danger of the mission, Spinney -- then in his 50s -- agreed. But officials at NASA quickly realized there was no way to bring the sizable Big Bird costume on board -- and so Spinney was replaced by a teacher.

That shuttle was the Challenger. "I remember watching the explosion on TV," he says. "Your scalp kind of moves when you see something like that."