Rob Reiner famously launched his filmmaking career with the 1984 mockumentary “This Is Spinal Tap.” But now, after four decades of features, including such generational touchstones as “The Princess Bride” and “When Harry Met Sally..

.,” he’s made a legit documentary. The subject is close to Reiner’s heart and has been since they first met as teenagers at Beverly Hills High School: Writer, director, actor, comedian and cultural shaker-upper Albert Brooks.

“It’s like with any documentary, fake ones or real ones, you’re basically writing the film with the pieces of film,” said Reiner during a recent phone conversation as he was getting “Spinal Tap II” ready for a friends-and-family preview screening (“I’m very nervous because I don’t know ...

I’m anxious to see how people respond to it.”) “There’s no script,” he continued of “Albert Brooks: Defending My Life” (a play on Brooks’ 1991 comedy “Defending Your Life,” with Meryl Streep). “It’s like putting together a jigsaw puzzle where there’s no picture on the box.

For me, it was a little easier because I know the picture is Albert. It’s all going to be about Albert. And I know Albert as well as anybody.

They always say, ‘How do you make a sculpture of an elephant?’ Well, you cut everything away that’s not an elephant. So I just made sure I cut away everything that wasn’t— “Is this a weight thing? What are you talking about?” Brooks, who is also on the call, .