The clip, which begins with an "In Memorium" notice, is anything but glum, illuminating the intimate and often-hilarious relationship the two developed later in life.
"That's what's good about losing your memory, there are so many surprises," Fisher teases Reynolds at one point.
The clip features jokes between the two, while also showing Fisher's concern for her beloved mother. Fisher shows real concern at Reynolds' continued determination to perform well into her 80s.
"My mother will forget she's not 35," Fisher says.
There's also plenty of ruminations about mortality and fame. Of Reynolds, one of Hollywood's greatest stars in the 1950s, her daughter says, "Age is horrible for all of us, but she falls from a greater height."
Fisher's past is also examined.
"Manic depressive was not diagnosed then," Reynolds says of her daughter, who had been open in the past about her battle with mental illness. "So, nobody knew what was going on with Carrie," she adds as old home movies are shown.
Fisher admits, "I went too fast, I was too much, I couldn't handle it.”
In the end, however, the documentary is about family.
Fisher is heard to say as the clip comes to a close, "I'm my mom's best friend. … Far more than I ever would want to, I know what my mother feels and wants. ... There's a lot of it."
Fisher and Reynolds will be buried in a joint funeral later this week in California, a source close to the family told ABC News Tuesday.
"Bright Lights: Starring Carrie Fisher and Debbie Reynolds" premieres on HBO Saturday.