I just want to get these similarities in a list. For the ones who have seen the two movies, it is interesting, and people who have seen only one of these two, it will certainly make them interested in the one they haven’t seen.
For the ones who haven't heard about a mockumentary: http://en.wikipedia.org/wiki/Mockumentary
Because of these resemblances between the two, I don’t really believe that Anvil is real: some parts of the movie must be acted to make the connection between the two.
First of all, short introduction to the two movies.
This is Spinal Tap
This Is Spinal Tap (officially spelled This Is Spın̈al Tap, with a non-functional umlaut over the letter n — n-diaeresis — and a dotless letter i) is a 1984 mock musical documentary directed by Rob Reiner about the fictional heavy metal band Spinal Tap. The film satirizes the wild personal behavior and musical pretensions of hard-rock and heavy-metal musical bands, as well as the hagiographic tendencies of rock documentaries of the time.
More about this movie: http://en.wikipedia.org/wiki/This_Is_Spinal_Tap
Anvil! The Story Of Anvil
Anvil! The Story of Anvil is a 2008 documentary film about the Canadian heavy metal band, Anvil. The film is directed by screenwriter Sacha Gervasi, whose previous credits include The Big Tease (co-written with Craig Ferguson) and The Terminal.
More about this documentary: http://en.wikipedia.org/wiki/Anvil!_The_Story_of_Anvil
Second: Respect to Anvil
When Spinal Tap philosophizes, discusses about music, instruments, it is meant to be funny; they are really “dumb”. When Anvil talks, it is not funny; it is real. When they are funny, it is because they do something funny, not because they are dumb. So that is the big difference. The guys just have bad luck: so don’t think that the anvil guys are as dumb as the not-existing guys from Spinal Tap.
Also the manager of Anvil, booked afterwards a part of the tour of "The Scorpions".
The resemblances: SPOILER ALERT.
1. First of all they both started as a band, different from Heavy Metal. Anvil started in the seventies, Spinal Tap in the sixties. Anvil started as “lips”, that was later the nickname of the singer. Spinal Tap started as something else: “the originals” (not going to spoil all their names, because it is a good joke in the movie)
Most bands begin with another name. But they both started from a softer genre before going into Heavy metal.
2. The lyrics. Both bands have funny ambiguous lyrics.
Spinal Tap had problems with the album cover because it was sexistic. Anvil had also a problem for being sexistic: but it were the lyrics.
Note: One of the most funny things about the album cover in Spinal Tap is that they changed it into a fully black cover and how they afterwards explain the meaning of it. The philosophy behind it. Reminded me of a documentary I saw about "The Black Album" of Metallica when they are discussing the also black album cover of “The Black Album”. (The black album came out 6 years later, so it was not to laugh at metallica)
3. In both movies the two leaders of the bands go to a fast-food restaurant. Talking about how they began. And in both movies they begin humming and singing an old song they made back in the beginning of their career. In Anvil it is just a couple of seconds, while in Spinal Tap it is a long scene. But for the rest this scene is perfectly identical.
4. While Anvil, from America, do their first big tour to Europe in years. Spinal Tap, from the UK, can do their first big tour in years in America. Both tours have big problems.
5. They both get a new manager that is female. And they both make mistakes.
6. Spinal Tap: in the hotel they see another famous musician who has success. They talk to him, thinking that he still knows them. But he is “so famous” he hardly recognized him. In Anvil, Lipps’ is backstage talking to many famous guys. Some of the guys recognize Lips (like twisted sister’s J.J. French). But still there is a resemblance: especially with Carmine Apice. Who does not really react to Lips’s story. He's not interested in his story, and doesn't recognize Lipp's.
6. They both take the train for touring when they have the female manager. (Little thing)
7. Anvil had nobody at a concert, because there was no good promotion for it. In Spinal Tap there was nobody at a signing session because of the promotion.
8. Spinal Tap talks in the beginning constantly about “the new album”, and they have problems getting the album out. Anvil also talks constantly about “the new album”, and they also have problems getting the album out.
Reminded me also of Metallica's "Some Kind Of Monster" documentary. When they are constantly talking about how good the new album is, how it will change their career.
9. In both bands one of the band members goes away and comes back.
10. Spinal Tap wants to make a Stonehenge decor for their new tour, they have a song about Stonehenge. So they are little bit obsessed with Stone Henge. When Anvil is touring the UK they want to visit Stonehenge, and they are also obsessed with Stone Henge.
11. Both films end in Japan (not going to spoil this one). Just the ending is identical.
There are maybe more. But these are the ones I first found.
Also found an interview with anvil talking about Spinal Tap: