Historical Errors made in Caligula, The Movie
I want to break this section up in to two parts, the actual errors in the script written by Gore Vidal, and then the main characters in the movie, and thier actual place in Roman History. First, I want to list the 19 main characters in the movie, below that will be the errors in the movie.
Gore Vidal's Characters:

Caligula-Rome's 3rd Emperor

Drusilla-Caligula's Sister

Agrippina-Not Stated

Caesonia-Caligula's wife

Julia Drusilla-Caligula's daughter

Tiberius-Rome's 2nd Emperor

Nerva-Tiberius' friend

Macro-Head of Praetorian Gaurd

Chaerea-Macro's replacement

Ennia-Macro's wife

Longinus-Imperial Treasurer

Gemellus-Grandson of Tiberius

Charicles-Caligula's doctor

Messalina-Not Stated

Mute Giant-Caligula's Bodyguard

Livia-wife of Proculus


Claudius-Uncle of Caligula

Mnester-Not Stated
Actual Roman names and descriptions:

Gaius Caesar "Caligula"-Third Emperor of Rome 37-41 AD

Julia Drusilla-Elder sister of Caligula, 11-38 AD

Agrippina the younger-Younger sister of Caligula, born 15 AD, she
was banished in the fall of 39 AD.
Milonia Caesonia-Caligula's 4th and final wife, born in 5 AD, and
murdered with him in January 41 AD
Julia Drusilla-Caligula's daughter, born in the fall of 39 AD, and
murdered with her parents in January 41 AD
Tiberius Claudius Nero Caesar-2nd Emperor of Rome, 14-37 AD. His
son Tiberius Drusus was the father of Gemellus.
Cocceius Nerva-was a Senator who expertise lay in the knowledge
of the law. He was known to have committed suicide by starvation.
Quintus Naevius Cordus Sutorius Macro-Prefect of the Praetorian        Guard under Tiberius. Committed suicide in 37 AD.
Cassius Chaerea-Replaced Macro in 37 AD as the Prefect of the
  Praetorian Guard. He led the assination in 41 AD.
Ennia Thrasylla-Wife of Macro, comforted Caligula after the death
  of his first wife, Junia, about 34 AD. Committed suicide 37 AD.
Lucius Cassius Longinus-was actually the first husband of Drusilla,
  they were later divorced. He was not the Imperial Treasurer. I'm
  trying to find out the name of the actual Imperial Treasurer.
Tiberius Gemellus-Grandson of Tiberius, forced to commit suicide
  in 37 AD, just before Macro and Ennia had to do the same.
Charicles-Doctor to Tiberius, and Macro's agent.

Valeria Messalina-was a 2nd cousin of Caligula. She was born about
  25 AD, and married to Claudius in 40 AD.
Sabinus was a large German who led Caligula's bodyguard, he            survived Caligula's assasination.
Livia Orestilla was the 2nd wife of Caligula. She was forced to           divorce her husband, Calpurnius Piso in 37 AD.  Caligula divorced
her after two months because he said she was barren.
Scribonius Proculus-He was a senator who was involved in the first assassination plot on Caligula, and was killed in 40 AD.
Tiberius Claudius Drusus Nero Germanicus-The 4th Emperor of
Rome. He was chosen by the Praetorian Guard under Chaerea.
Mnester-He was an actor, and also a lover of Caligula.
Error's in Gore Vidal's Script for Caligula
Gore Vidal seems to have based his script a lot on the 1934 fictional novel by Robert Graves called "I Claudius". I have tried to go by the sources, Dio, Suetonius and Tacitus, as well as the 2 excellent books by Anthony A. Barrett, "Caligula: The Corruption of Power", and "Agrippina". Here's some obvious errors in the script of Caligula.

1. The incestious relationship between Caligula and Drusilla was mentioned once by Suetonius as gossip by the grandmother, Antonia, and should not be taken seriously.

2. Drusilla was not married to Marcellus. Her first husband was Lucius Cassius Longinus, married in 33 AD, and from whom she divorced in 37 AD. Her second husband was Marcus Aemilius Lepidus, whom out-lived her, but not Caligula, who had him killed in the fall of 39 AD.

3. Ennia, the wife of Macro, became Caligula's lover after his first wife, Junia, died in 34 AD. Caligula was called to Capri in 31 AD, so there is a time frame problem in the script, as Caligula didn't have a thing with Ennia until 3 years after he had been in Capri.

4. The omen with the Black Bird at Capri was an invention of Gore Vidal, as well, in that scene it seems like they are back in Rome, then in the next shot when Nerva dies, they are at Capri? Continuity is a problem that plagues the movie! The black bird Omen is repeated towards the end of the movie, but that time is scares Caesonia.

5. Nerva actually starved himself to death, in an effort not to have to serve under Caligula. 

6. In Capri, Tiberius only had supporters, so the scene with the three men in the red mud bath probably would not have happened. The three men put down Tiberius, and Caligula witnesses it from afar, and I mean afar! It was fact that Tiberius had supporters who would try to trick people into saying bad things about the Emperor, then run right to him with the information. Many times Caligula was tested this way on Capri, and he never had a bad word to say about him, even though he killed most of Caligula's family.

7. Longinus was not the Imperial Treasurer or Caligula's Financial Wizard, he was really a nobody from the Senatorial Class (and Drusilla's first husband). I'm still trying to find out if Caligula really had such a person at his side.

8. Macro was the one who told Caligula to be warey of Gemellus, not Caesonia. Macro killed Tiberius March 18th 37 AD. (For the record).

9. Although Caligula was not actually "betrothed" to Ennia, the wife of Macro, Caligula did promise to marry her if he became Emperor. This was just coming off the death of his first wife Junia, and the loss of thier child as well. So Caligula was emotionally weak. Once Caligula decided to get rid of Macro, Ennia was no longer in the picture. In the movie, Caligula killed Macro via the "Killing Machine" and Ennia was banished. According to history, they both committed suicide to protect thier estates.

10. I have not come across any source yet that can verify the "Killing machine" that Caligula uses to dispose of Macro in the movie. To the best of my knowledge, this was a figment of Gore Vidal's imagination.

11. Historically speaking, Gemellus died before Macro. He was arrested for taking an antidote, and he was forced to commit suicide. He was given a sword, and told to do the job. This happened after Caligula recovered from his illness, about October 37 AD.  The killing of Gemellus was in defence of a plot to make him Emperor, Macro suspected that something was up, and the call was made. It was said that Silanus, Caligula's former father in law, had plans to support Gemellus as Emperor. Silanus died soon after Gemellus, and then Marco and Ennia's deaths followed in early 38 AD.

12. Livia and Proculus were real people from Caligula's time, but Gore Vidal changed thier actual postions in society for the movie. Livia Orestilla was actually the 2nd wife of Caligula. She was already engaged, to Gaius Calpurnius Piso, and was forced to marry Caligula towards to the end of 37 AD. Caligula saw her, much as he did in the movie, took a fancy to her, and decided she would be his wife. But, within 2 months, he had tired of her, and had her banished. Her husband, Piso, was still alive by June of 40 AD. The Senator Scribonius Proculus, was part of the first assassination attempt on Caligula, and was smoked out during an investigation in 40 AD. He was killed by Senators, who even though they hated Caligula, wanted to prove he had nothing to worry about. The death of Proculus set back the actual assassination about 6 months. Proculus in history was not a defender of Caligula as the movie would have him be. As well, as far as history is concerned, Proculus and Livia probably didn't really even know each other.

13. Caligula's sister, Julia Drusilla, died on June 10th 38 AD, at about 22 years of age. Caligula did not meet Caesonia until the spring of 39, and thier marraige took place that summer, with the birth of thier daughter, Julia Drusilla, in the summer or autumn of 39 AD. So there is no way that Drusilla and Caesonia could have been in any contact as they do in the movie.

14. From what I have read, I have not seen any historical references that have shown Caligula wanted a son so bad he was convinced that his daughter was a son, as shown in the movie. He was however, very devoted to the child.

15.  Again, just another mis-order of events by Gore Vidal. Gore's script has this order of events, Macro's death, meeting Caesonia (thru Drusilla), Gemellus' death, Caligula's sickness, Caligula's marraige and birth of his daughter, Drusilla's sickness and death. In actuality, it went as such, Caligula's sickness, Gemellus' death, Macro's death, Drusilla's death, then meeting and marrying Caesonia, then Julia Drusilla's birth.

16. Although I don't think it was Proculus, I do recall reading the story about Caligula having someone's penis cut off and fed to the dogs. I will have to re-read some material and try and find where I read that.

17. During the "battle of Britain" in 40 AD, Caligula actually collected sea shells to bring back to Rome, not Papyrus as shown in the movie.

18. Although Chaerea did kill Caligula, he was not supportive of Claudius as Emperor.  Because Caligula was still popular with the general public, Claudius could not stand behind regicide. He replaced Chaerea and forced him to commit suicide. I'm not sure what Chaerea's plan was after he killed Caligula, but his soldiers did not support his stand against Claudius.   

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