There are not enough movies solely about bass players. Luckily, most of the ones we do have are quite good. These are some of my favorite movies/documentaries which focus heavily on bassists.
Standing in the Shadows of Motown
James Jamerson is one of the early pioneers of the electric bass. He was the studio bassist for Motown records and played on countless hits. This excellent documentary is about Jamerson and his fellow studio musicians. Together they were The Funk Brothers.
While there were many bass-playing developments after Jamerson, I'd argue that in terms of note choice and rhythms few have improved upon what he did. And, Jamerson did it with few people to copy. It's important to understand his genius from that perspective. Where did such advanced playing come from?
I highly recommend getting the book Standing in the Shadows of Motown as well. If you study it well, it has everything you need to know about creating basslines in it.
This movie is required viewing for you.
Carol Kaye: Session Legend Interview
Carol Kaye is another pioneer bassist leaving an indelible mark on music and bass history. Carol Kaye was part of the "Wrecking Crew"—a group of L.A. studio players who played on thousands of recordings. Many bands didn't play on their own recordings. Studio time was expensive, and it was more economical to bring in incredible players who could lay anything down in one or two takes. Listen to this lady.
This excellent documentary, by Phish bassist Mike Gordon, is about Allman Brothers and Gov't Mule bassist Allen Woody. I can't think of another movie with this many great bass players in it. All bassists will get a lot out of this one.
Jaco Pastorius is often described as the greatest bass player of all-time. In this documentary, brought to you passionately by bassist Robert Trujillo, you will learn about Jaco's life, his playing and his tragic end. A must watch for every bass player.
Charlie Mingus: Triumph of the Underdog
Bassist Charlie Mingus is one of America's greatest composers. This documentary (and the next) will give you insight into this phenomenal musician and creator.
Mingus: Charlie Mingus 1968
Another excellent Mingus documentary. Learn about this man.
Percy Heath Interview
Percy Heath, bassist for the Modern Jazz Quartet, gives a great interview on playing bass and about his experiences playing with MJQ and many jazz greats. You might not realize it for a while, but he tells you everything you need to know right here; you just need to fill in the details. An important bass and music document.
Cachao: Uno Mas
Bassist Israel "Cachao" Lopez is one of Cuba's most important musicians. Cuban music is unique and incredibly rich. Cachao's impact took the world by storm with the creation of the mambo. This PBS documentary is hard to find, but worth the effort.
An Ox's Tale: The John Entwistle Story
John Entwistle, bassist for The Who, is an important rock bass pioneer. He brought the bass into the spotlight in songs like My Generation. His influence and inspiration are everywhere. Also, check out The Kids are Alright.
The Quiet One
Rolling Stones bassist Bill Wyman is another important rock bass pioneer. This documentary tells his life and presents his many photos and videos.
Every rock bassist needs to know Hawkwind and Motörhead bassist and singer Lemmy. This documentary is a very intimate portrait of the man.
Metallica: Cliff 'Em All!
This is a tribute to the great metal legend Cliff Burton. There aren't enough clips of him speaking, but there are some excellent clips of early Metallica shows and bass solos.
New York Doll
A documentary on the life of New York Dolls bassist Arthur "Killer" Kane. This film details the influence of the New York Dolls, Kane's struggles after the Dolls, his conversion to the Mormon faith, and putting together a reunion show.
Love & Mercy
A dramatic movie about bassist Brian Wilson of The Beach Boys.
Sid & Nancy
A film about Sex Pistols bassist Sid Vicious. This is more of a tortured love story than bass player documentary. You will be learning lessons from this movie, but they won't be all the fingerings for a major triad. Heartbreaking.