Why is Lex Luthor on film so boring? Maybe it's because he's given no time for his complex personality to come out. Let's compare the live-action Luthor to the animated Luthor, where the villain is given a HUGE amount of room for his character to come out.
What's interesting about Lex Luthor is that, for decades, he was NOT Superman's nemesis in the way Joker was to Batman. In Luthor's first appearances in the 1940s, he was variously the dictator of a small European nation hoping to spark a new world war (once the great powers had destroyed themselves, he'd take over the world), a super-scientist turned evil, or an evil corporate businessman. He did not hate Superman; instead, he merely knew he had to have some plan to deal with the Man of Steel in order to allow his nefarious plans to proceed.
In the early 1950s, comic book writers finally settled on Luthor as sort of genius con-man -- someone whose genius scientific schemes were designed to bilk the populace out of money or power. Luthor disappeared completely in the mid-1950s. In the late 1950s, he began to show up again. Now the con-man aspect was dropped and Luthor became a Superman-hating super-scientist. In both periods of the '50s, Luthor was portrayed as rather fat. Dressed in a business suit in the early 1950s, he was almost uniformly portrayed wearing prison greys in the late 1950s.
In Adventure Comics #271 (April 1960), Jerry Siegel finally gave Luthor a backstory: He was a childhood friend of Superboy's. Superboy saved his life during a laboratory fire, but Luthor was convinced that Superboy was jealous of Luthor's genius. In Superboy comics, Luthor repeatedly tried to trick Superboy into doing damage to Smallville, so that Luthor could come to their rescue and they'd love Luthor rather than Superboy. In Superman comics, Luthor began to evolve: By Superman Vol 1., #164 (October 1963), Luthor had slimmed down and muscled up and become the modern visual incarnation people know today. Other than his appearances to attack Superman, Luthor was not shown to have any sort of life. The means by which he financed his capers, or developed his technology, was never explored. (That was a common flaw in the 1960s.)
In Superman Vol. 1, #282 (December 1974), Luthor became a super-powered supervillain in his own right. He got a purple and green flight-suit and could fly using tiny jets in the boots. At times, the suit appeared to give Luthor super-powers similar to Superman's. At other times, Luthor merely incorporated high-tech devices into the gloves that allowed him to take on Superman. (In issue 282, for example, Luthor's gloves contain both a de-aging ray and a "gravity-caster" ray that make Superman super-heavy so that he cannot fly.)
For Superman's 45th anniversary in 1983, Luthor was given a makeover. DC Comics had licensed its characters to Kenner, which was producing a SuperPowers toy line of action figures. George Perez jettisoned Luthor's decade-old togs in favor of a bulky, purple and green "warsuit" that carried a load of kryptonite-powered weapons. The warsuit debuted in Action Comics Vol 1., #544 (June 1983).
Lex Luthor FINALLY gets a life in Superman Vol. 1, #416 (February 1986). He's shown to have formed LexCorp, a massive congolmerate which dominates economic life in Metropolis. LexCorp is completely legit, although as majority stockholder Luthor uses the profits he makes to fund his schemes. Luthor also occasionally invents things for LexCorp to allow it to make money. (This concept would be greatly expanded in John Byrne's The Man of Steel Superman reboot limited series, which ran from July to September 1986.)
Luthor actually DIES in Action Comics Vol 1., #670 (October 1991). The kryptonite ring he's worn for several years has given him cancer. A new character, Luthor's illegitimate son Lex Luthor Jr., appears. Two years later, readers learn that this is a clone of Luthor's original body, with Luthor's brain implanted inside (Superman Vol. 2, #77 [March 1993]). Luthor is defeated in his plan, and ends up in a vegetative state again ("locked in"). He regains the use of his body by making a pact with the demon Neron in Superman: Man of Tomorrow #3 (December 1995), and manages to get himself acquitted of all previous crimes by framing Cadmus Labs (Action Comics Vol 1., #737 [September 1997]).
This takes us up to Superman: The Animated Series, which began airing in 1996.
Between 1986 and 1996, Luthor appeared in 140 Superman stories. (That's 45 times in Action Comics, 3 times in Superman Vol. 1, 46 times in Superman Vol. 2, and 45 times in Adventures of Superman.) There were a total of 468 Superman issues during this period. That means Luthor appeared in about a third of all Superman stories. Most of his appearances, however, were just a few panels. Only about a third of all Luthor stories during this period featured Luthor as the main character. (Example: In the Man of Steel limited series in 1986, Luthor is in two issues, but in neither is he the central character or central villain.)
I'm going on and on........... my point is, Luthor really had never been defined as a character when the Animated Series began. Extensive development of Luthor's character and backstory had only begun a decade earlier, and in truth very little of it had been explored. Most of the exploration of "Luthor as evil Bruce Wayne" had occurred in the five year period between 1986 and 1991.
So what makes the accomplishment of the Animated Series even more astonishing is how much they managed to do with Luthor in just four years. They completely fleshed out the character and included most of the major comic book stories, while giving Luthor a real life and personality.