1991's The Silence of the Lambs isn't really the type of movie to be subjected to fan theories - especially of the outlandish sort. It's faithfully adapted from the novel of the same name, which itself veers very close to reality and accurate crime reporting/research. Its characters are also quite realistic, resulting in a grounded movie that doesn't really lend itself to theories in the way, say, The Avengers would.
But, this being the internet, Silence of the Lambs has indeed become the subject of numerous fan theories. Some of these are obviously ridiculous and in no way could be considered legitimate. Others are definitely more convincing. The movie may not lend itself to theorizing, but the fans have done so, and they have come up with some brilliant suggestions.
Far Fetched: Hannibal Lecter Is A Sith Lord
Much has been written regarding the timeline and continuity of the Hannibal Lecter films. But what most people will agree on is that The Silence of the Lambs does not take place in the same universe as Star Wars.
This theory can be found over on TV Tropes, stating, "Because, come on—HOW in Hell did he get ahold of that pen if he didn't use the Force?" It's a funny suggestion because the movie never explains how Hannibal got his hands on the pen. But it can be stated with certainty that he is not a Sith Lord.
Makes Sense: Manhunter Is In A Different Universe
Now what can be agreed upon is that Michael Mann's Manhunter takes place in a different universe than The Silence of the Lambs. Manhunter was released in 1986 and serves as an adaptation of Thomas Harris's Red Dragon.
As the theory convincingly points out, "Chilton isn't a d***. Dollarhyde is killed by Will instead of by Molly. Will's marriage doesn't fall apart. Hannibal's victims were co-eds who happened to be his patients. His last name isn't even spelled the same (Lecktor). Heck, the Manhunter Lecktor might not even have been a cannibal at all."
Far Fetched: Lecter Is A Time Lord
And once again, there is a theory that Hannibal Lecter is some type of fantasy being. According to this theory, "If we somehow incorporate the theory below, Aaran Thomas regenerates into Gaspard Ulliel regenerates into Brian Cox, who regenerates into Anthony Hopkins."
Furthermore, "Perhaps Hopkins regenerates into Mads Mikkelsen, making him the Fifth Lecter?" (a reference to the TV show Hannibal). It's certainly a fun theory, but then again, this point could be made for any remake or sequel with a replaced actor.
Makes Sense: Separate Continuity From Hannibal & Red Dragon
This is similar to the Manhunter theory, but it contains one key difference - both Hannibal and Red Dragon star Anthony Hopkins, making it clear that they share a timeline with The Silence of the Lambs. However, the theory makes some good points.
For example, "Jack Crawford is portrayed by Harvey Keitel in Red Dragon and by Scott Glenn in The Silence of the Lambs." Furthermore, "In Hannibal, the dialog in the recorded conversation between Lecter and Starling is slightly different from how it is heard in The Silence of the Lambs." Finally, and most importantly, "At the end of Red Dragon, Dr. Chilton goes to Lecter's cell and tells him that a female FBI agent is there to see him. This is clearly meant to be Clarice Starling. However, in The Silence of the Lambs, Chilton does not leave Starling to see Lecter in his cell before she visits him, and his clothes are different."
Far Fetched: Lecter Is A Genetic Experiment
Apparently, the fans of The Silence of the Lambs really believe that Hannibal Lecter is some type of inhuman being.
This bizarre and far-fetched theory posits that "Hannibal Lecter is a genetic experiment in making a superman that went horribly wrong, accounting for the maroon eyes, the Improbably High I.Q. and six fingers on one hand. He will later change his name to Khan Noonien Singh and lead his favorite bits of humanity in killing the rest in the Eugenics Wars." So, not only does Silence take place in the Star Trek universe, Hannibal Lecter becomes Khan!
Makes Sense: Long Con
According to this surprisingly decent Reddit theory, Hannibal Lecter manipulated Buffalo Bill in the hopes that he could get him out of prison. The theory posits that Hannibal saw the psychotic traits in Bill, and rather than curing him, he spurred him into becoming a serial killer.
Therefore, should Hannibal ever be caught and sent to prison (which he obviously was), the authorities would be forced to go to Hannibal for information on Bill (which they obviously do). Hannibal could then use this information to finagle a better deal and/or escape prison (which he does). Bill was nothing but a long con and a pawn in Hannibal's potential prison escape.
Far Fetched: The FBI Creates Serial Killers As Assassins
This Reddit theory is well posited, but ultimately unconvincing. It reads a bit like a conspiracy theory, arguing that the FBI creates serial killers to use as assassins.
According to them, "Demme implies that the FBI doesn't CATCH serial killers, it CREATES them to use as covert assassins. It employs trauma-based mind control to conjure these covert agents of death. Multiple (Personality Disorder) Miggs, Buffalo Bill and Hannibal the Cannibal are all former pupils of the program - and Clarice is next in line for the treatment."
Makes Sense: Faustian Deal
Another solid Reddit theory posits that The Silence of the Lambs is some type of modern retelling of the Faustian legend. The theory is well worth reading, arguing that Hannibal Lecter is the devil and that "Clarice sells her soul to him in order to stop Buffalo Bill in the form of sharing her secrets."
There's a lot more to it than that, including some rich symbolism involving the lambs and Hannibal's violent escape from prison. There's a lot to like here, and it would make a great English essay.
Makes Sense: The Batman Is Influenced By SOTL
The Batman has not been released yet, but according to this Reddit theory, The Silence of the Lambs plays an enormous influence on its creation, plot, and general aesthetic.
This includes, "A scene in The Batman is a direct homage to a scene in SoTL," "The plots feature clear parallels," and "The film is taking influence from the classic Batman comic The Long Halloween, which also references SoTL." There's a lot more to it than that, and the theory itself is well worth reading and regarding.