"Heat" is arguably everyone's favorite Michael Mann movie.

It's the source of a rare Robert De Niro/Al Pacino summit, one of the most gripping cops-and-robbers thrillers ever made, and a quintessential Los Angeles movie. Yet, when it opened 20 years ago (on December 15, 1995), it went unheralded by the Angelenos in the Academy.

Despite being snubbed for Oscar nominations, it went on to be considered a classic, one imitated not only by filmmakers (ahem Christopher Nolan's "Dark Knight") but also by real-life crooks. Still, for all the obsessive viewings and re-viewings, there's a lot you may not know about "Heat." Here are 11 things you need to know.
1. The inspiration for De Niro's Neil McCauley was a real-life Chicago thief of the same name. The real-life Vincent Hanna (Al Pacino's character) was Chicago cop Chuck Adamson, a technical adviser on Mann's TV police dramas "Miami Vice" and "Crime Story."

2. As in the film, Adamson and McCauley actually did have a sitdown at a coffee shop, where they treated each other with the mutual respect of disciplined professionals. Later, Adamson's team did kill McCauley in a shootout after a robbery.

3.Keanu Reeves was initially attached to the role of Neil's right-hand man, Chris. But Val Kilmer stepped into the part when he became available during time freed up from his "Batman Forever" shoot.

4. Mann spent more than a decade developing the "Heat" script. In 1989, he shot an abbreviated version of it as the TV movie "L.A. Takedown" (pictured below). It was made as a backdoor pilot to a possible TV series about Los Angeles cops. NBC declined to pick it up as a series.
5. The full "Heat" script found its way to producer Art Linson, who gave it to De Niro and Pacino. Though the two iconic actors had both appeared in 1974's "The Godfather Part II," they had never shared screen time. Of course, in the three-hour "Heat," they'd only have about 10 minutes of scenes together.

6. The research the cast and filmmakers did on the lives of cops and criminals was extensive. Mann spent seven months going on ride-alongs with L.A. robbery and homicide detectives. Those playing McCauley's gang met with real-life convicts and ex-cons, including Danny Trejo, who ended up landing a role in the film.

7. The preparation for the heist and shootout that make up the film's central set piece was the most meticulous of all. Actors playing both cops and robbers trained with weapons for three months on a police shooting range, with makeshift buildings and cars added to simulate the layout of the actual downtown Los Angeles location. Tom Sizemore and some of the other thief actors, wearing disguises and body armor, went to the bank and cased the joint. Producers had tipped off the bank's manager, but the tellers and guards were none the wiser.

8. The actual shoot of the bank robbery took several weeks to film because the production had access to the location only on Saturdays and Sundays. Each take resulted in the firing of 800 to 1,000 blank rounds.

9. The sound the gunshots made as they echoed off the walls of neighborhood skyscrapers was so terrifying that Mann used them as they were recorded, rather than dubbing in sound effects in post-production.
10. The De Niro/Pacino coffee sequence was shot at Kate Mantilini, a Beverly Hills late-night restaurant owned in part by Harry Lewis, one of Humphrey Bogart's co-stars in the classic crime thriller "Key Largo." Visitors to the eatery liked to request seats at the table where the scene was filmed. Alas, the restaurant closed in 2014.

11. The budget for "Heat" was reportedly between $50 and $60 million. It earned back $67 million in North America and another $120 million overseas.