In a scene where a bunch of people are fighting to get onto an elevator, the extras were really fighting to keep off the elevator, since anyone who did get on it was pretty much done, and anyone who stayed off it was guaranteed a job for the next couple of months.
In the scenic elevator scene, the explosions were bigger than the actors were anticipating, so they really were panicking.
The director fired a handgun into the air to get a genuine look of surprise from the actors' faces.
Life Imitates Art: A real fire broke out on set, and Steve McQueen found himself briefly helping out real firefighters. One of them didn't recognise him and said "my wife isn't going to believe this", to which Steve responded "neither's mine."
Science Marches On: Since the making of this film, there was the notorious MGM Grand Hotel fire when it was discovered and publicized once and for all that in a major structure fire, it's not the flames themselves that is the major killer, it's the smoke.
Significant Anagram: Meta. "The Towering Inferno", as is often noted, is an anagram of "not worth fire engine".
Actually probably less than it might have been. Irwin Allen made sure the men's hair wasn't that long and the fashion wasn't too loud.
Wag the Director: Steve McQueen, a veteran of this trope by then, requested that he and Paul Newman have the exact same amount of lines in the film. Also he insisted that he not appear until forty three minutes in, by which point Newman would have used up a lot of his lines. William Holden attempted to - demanding he be billed first. But as he'd been in a few flops lately, he gets third billing.
Katharine Ross and Raquel Welch were considered for the role of Susan. Natalie Wood was approached too, but declined as she was pregnant at the time. She also found the script "mediocre".
The original script gave the fire chief a smaller role and was offered to Ernest Borgnine and Steve McQueen offered the role of the architect. McQueen insisted on playing the fire chief and thus the role was expanded.