This Sunday's episode -- entitled "Fearless" -- features Chapman as a music teacher who exhorts his students to play "fearlessly" as they strive for excellence. His ideals are put to the test when he is diagnosed with amyotrophic lateral sclerosis (ALS), commonly known as Lou Gehrig¹s disease.
What makes the episode remarkable is that veteran TV writer Ernie Wallengren, who was recently diagnosed with ALS -- a progressive neurodegenerative disease that attacks nerve cells in the brain and spinal cord, wrote it.
Wallengren was a regular writer on the long-running family drama "The Waltons." He also wrote for "Touched by an Angel," "Little House on the Prairie," "Eight Is Enough" and "Flipper" -- as well as the more adult oriented "Baywatch" and "Falcon Crest."
He said writing "Fearless" was a lot harder than he thought it would be.
"I thought it was going to be a piece of cake," he said. "There were days when it was incredibly difficult. I just couldn't face sitting down and writing it. It ended up being a very cathartic experience for me and allowed me to face a lot of the personal issues involved in this diagnosis."
"Doc" producers Dave Johnson and Gary Johnson ("High Incident," "The Client," "Second Noah") said Wallengren's determination to use his personal experience to heighten awareness of ALS was an inspiration. Dave Johnson said it is not uncommon for TV writers to draw on personal experiences -- even some that might seem a bit too personal -- for storylines.
"Those are the best stories, almost always," he said.
Johnson said the show's production staff took pains to make the episode as medically accurate as possible.
"We have a medical advisor who's always on the set," he said, "and we had the North American ALS Foundation check through everything to make sure everything was done right."
The episode featured some ALS patients in scenes set at support groups for people at various stages of the disease.
Johnson -- a TV veteran -- is impressed with the acting chops of Chapman, a novice in the acting game.
"He's fabulous," said Johnson. "It was a big risk, people thought, for me to give him that job."
Chapman may be new to acting, but he is an entertainment business veteran, with seven million albums sold -- five gold, two platinum -- and four Grammy Awards. He is a leading practitioner in the growing field of contemporary Christian music, and has won 45 Dove Awards.
Chapman said he felt pretty good about his work.
"The critics haven't made their official analysis," he said, "but I was real pleased with how it came out."
Chapman said he was nervous on the set, something he hadn't experienced in music for quite some time.
"Every album there's some anxiety," he said. "Are people going to like this? Are people going to like me? But after so many years I get to where I know this is what I can do. Acting is a whole difference experience."
Chapman said he's had "a little bit of a bug" for acting ever since appearing in school plays in the fifth grade. He got the role in "Doc" in much the same way as many roles are assigned in Hollywood -- networking.
He had met Dave and Gary Chapman about doing music, and possibly playing a role, in another project. They set him up with a screen test, and subsequently thought of him for the lead in "Fearless."
At first, he said, he was reluctant to take the role.
"I said the only way I'll do this is if it makes the show better," Chapman said. "I don't want to be some rookie wannabe actor if it hurts the show."
Mary Lyon, vice president, patient services, at the ALS Association (alsa.org) in Calabasas, Calif., said TV shows and movies about ALS have a spotty record for accuracy.
"There hasn't been one that we haven't gotten at least some patients or families who felt that it wasn't a good portrayal, for a variety of reasons" she said. "I don't know that any one movie or TV show is going to speak to the entire ALS population. But for anyone new to the disease, it is important to have portrayed for you what may lie ahead for your family."
Lyon said most shows she has seen tend to show the impact on the patient, not necessarily on the family. She said "Tuesdays with Morrie" did a good job of showing the impact on caregivers.
"The emotional and psychological impact of the disease is so overwhelming that physical and emotional depression is so easy to understand," she said. "Everyone in the family becomes affected in one way or another. Whatever future and dreams the family had suddenly become turned upside down."
"Fearless" focuses on the early stage of a man's disease. Wallengren hopes subsequent episodes will examine the progression of the disease.
"This is a story we have only begun to tell," he said.
Johnson said he too wants to do more episodes.
"No concrete plans yet, but we will definitely bring this character back at some point," he said. "It depends on Steven's schedule and our schedule."
It might depend on more than that. During an interview to promote "Doc," when he was informed that some ALS advocacy groups support embryonic stem cell research, he wasn't sure what to make of it.
"This is the first thing to that end that's even been mentioned," said Chapman, a committed Christian. "There's so many potentially sticky sort of messy things when I step out onto a platform and connect myself with some cause that's important, because there's just way of knowing all the things that are under the surface. You could connect dots from here to there and eventually find yourself in some discussion that could be real uncomfortable and I could make a stand on something I never expected to do.
"I'll probably have to handle those things as they come, if they come," he said. "At the core. Lou Gehrig's disease is a terrible disease. When people suffer that way, if my involvement with this TV show can bring some awareness some attention, maybe in doing so it could educate people on the disease."
Chapman also wondered whether he would be up to the acting challenge of playing the character in advanced stages of ALS.
"We all could go through and list the otherwise successful musical careers that were crippled if not ended by bad acting," he said.
It has been estimated that about 30,000 people in the United States have ALS, but Lyon said reliable statistics are hard to come by.
"There's not a reporting requirement for ALS," she said. "It's not communicable. The information we have is from population studies that have been done. We certainly know that people are not always diagnosed, or they get lost in the system, and don't get into the mainstream of care."
Wallengren said there is no treatment for ALS, just one FDA-approved medication that "delays the time of death by about three months for the average patient."
He gets around using lifts, wheelchairs, ramps and other technologies but eventually he will have to decide whether to go on a mechanical ventilator -- the kind that allows actor Christopher Reeve and scientist Stephen Hawking to breathe. He doesn't know what he will choose when the time comes.
"And I probably won't know the answer until this period is behind me and I'll be able to assess it," he said. "I do view it as quite an adventure. I'm having way too much fun to consider letting the disease win. I don't think I will be nearly as prolific. I've worked on and written hundreds of hours of television. I just plain can't type that fast anymore. At the very least I'm good for some mean haikus."
Celebrity Families of 2014 [PHOTOS]