The first word that comes to mind in attempting to describe 1979's Caligula is "prolonged". It's a stew with several good ingredients discernable but almost obliterated by the conflicting flavours of other things tossed in. It's a film with notorious extended scenes of hardcore sex that is at the same time often difficult to pay attention to. It's sort of like if someone played Led Zeppelin really loud at the same time Duke Ellington was being played really loud. It's almost dumb fate that the two pieces of great music occasionally coalesce into something interesting.
It actually resembles 1963's Cleopatra in the sort of mess it is, coming from feuding, multiple directors and screenwriters with conflicting visions for the project. I rather like to imagine the Caligula in this film played by Malcolm McDowell as being the grandson of the Octavian played by Roddy McDowall. Wow--I didn't even realise until I typed it out they have almost the same last name.
There is an impressive cast in Caligula--Peter O'Toole and Sir John Gielgud appearing in an expressionistic orgy sequence at the beginning as Tiberius and Nerva respectively. This expressionism in sets, lighting, and costume is contrasted with enormous, expensive looking realistic sets and locations in other scenes. Both are actually really well put together, the mild tonal dissonance not quite rising to the level of a flaw. The dissonances that most hamper the film are the extra porn scenes thrown in by Bob Guccione and a general lack of direction in the way the characters are written.
Tinto Brass was nominally the director for most of the shoot, but he left during editing and the contrast between his scenes and those added by Guccione are pretty severe. The best example I can think of is a scene where Helen Mirren, as Caligula's wife Caesonia, is having a three way with him and his sister, Drusilla, played by the beautiful and talented Teresa Ann Savoy.
This wasn't hot enough for Guccione, I guess, because he felt the need to include two women watching them through a peephole before being inspired by it to fuck each other, too.
Which is pretty good. It's some nice lesbian porn, though the actresses seem a little phoney. It's still decently shot and the girls are really beautiful. But it's like having a scene from Batman: The Animated Series spliced randomly into The Dark Knight.
Also coming to mind are hardcore bits shoved into a scene where Caligula observes a women's bath in disguise and a scene where he's prostituting the wives of senators. The behaviour of the porn actresses thrown in is a bit confusing. We see shame faced or stoic close-ups for older women, in the latter case, between shots of younger women apparently really getting their rocks off. Are these also senator's wives? Are they prostitutes? Did Caligula actually do something these wives had wanted him to do? I think the question that most often came to mind in these extraneous scenes was, "Who are these people?"
Which is not to say there's not hardcore stuff that was shot by Brass, and it's not to say I'm against hardcore sex in movies. It doesn't bother me in the slightest, nor does having it continually in the movie. Some might say, okay, once you've established that Rome at this time was filled with jaded, gratuitous sex and violence, that's enough. I would say--just because it's established for us doesn't mean it stops happening. This is the world the characters live in, and keeping it present is important. But what we have here is sort of like a wampa inexplicably showing up on a Star Destroyer.
The inconsistency in characterisation is a bigger problem. O'Toole and Gielgud come out well because they're not in the movie very long--Gielgud in particular has an interesting suicide scene, personifying the last vestige of a nobler Rome.
Helen Mirren is subtly interesting as an intelligent courtesan Caligula seems to really love and who genuinely seems to get him. But Caligula himself is a trainwreck of writing. We get scene after scene of him being decadent, and the idea of a man whose assumption of his divinity has made him reckless and lazy comes across. But it's prolonged--a few less scenes of him horribly slaughtering a helpless and loyal captive or having strange and violent sex would've improved the film. There are a few good notes that are held longer than the melody can support.