There's a bomb on a bus. Once the bus goes 50 miles an hour, the bomb is armed. If it drops below 50, it blows up. As movie premises go, this one is absolutely ridiculous, right? You'd have been forgiven for thinking so, at least, as few involved with Jan de Bont's "Speed," which was released by 20th Century Fox on June 10, 1994, could have anticipated its popularity. The film was a runaway hit, winning two Oscars and grossing over $350 million worldwide. Now, 20 years later, it's a celebrated relic of an era before blockbuster filmmaking was so awash in digital wizardry, an era when practical movie magic sold the highest of concepts to the masses.

For actor Keanu Reeves, who starred as the film's hero, LAPD S.W.A.T. officer Jack Traven, it feels like that long ago if only because so much has changed over the last two decades. Though he had already starred in Kathryn Bigelow's "Point Break," it was "Speed" that turned him into an action star Hollywood would test in films like "Johnny Mnemonic," "Chain Reaction" and "The Matrix" throughout the rest of the '90s. He looks back on the film today as a fond memory in the unassuming early years of his career.

"I think there's something that people respond to in the film in the sense that it feels so accessible and human, in a way," Reeves says. "There's a vulnerability to it. Having participated in that, and having had a great opportunity, and then to be here 20 years later, it feels like that came from a more innocent time."

It also came from a time when audiences only knew actress Sandra Bullock, if at all, from work in comedies like "Love Potion No. 9" or futuristic actioner "Demolition Man" opposite Sylvester Stallone. It was "Speed" that sent her career soaring, but, of course, she couldn't have possibly seen that coming.

"I don't think anyone had any idea what was going to happen with that film," the Oscar-winning actress says. "If someone says they did, they're lying — unless in the editing process they felt something come together. But I certainly didn't feel it. I think we were sort of ridiculed a bit for being the 'low budget bomb-on-the-bus movie.' Not that I cared. I was just so happy to have a job and that I got to work with Keanu. I was grateful no matter what it was."

Indeed, Bullock wasn't at all ideal for Fox at the time. The list of more established actresses who were up for the role is long and considerable, from Meryl Streep to Kim Basinger and all points in between. They all turned it down, unmoved by the outlandish concept. But director Jan de Bont fought for Bullock, who had the girl-next-door look and appeal that he felt the role of bus-riding graphic designer (turned bus-driving heroine) Annie needed.

"Initially every studio wants bigger stars for lead roles, and I understand that," de Bont says. "But I could not see Julia Roberts driving this bus. I could not see several other actresses. I would never believe they would ever even be on a bus. I felt I needed an actress who you could believe would have taken the bus, and Sandra had this kind of every day look – I mean that in a good way — in the way she dresses, the way she behaves, very casual."

The character of Annie was as important as Jack Traven, de Bont says, because "she had to keep this whole team together and keep the tension going. And responses — secondary reactions are really key in movies like this. How real are they? How believable are they? She did a fantastic job on that. She was exactly what I hoped for, and thank God the studio, at the very last moment, let me choose her."

The ever-affable Bullock remains indebted to the director, with whom she worked once more on the film's ill-fated sequel in 1997, for going to the mat for her. "He chose me over so many people that probably would have helped that movie get kicked off in a bigger way," she says. "He gave me the opportunity. So I've got to say, he had some pretty big balls. And I'm grateful for his large balls. And you can quote me on that. And if you can get a visual to go along with that quote, that would be great!"

Filling the bus with reality

De Bont was developing a film about sky-diving at Paramount Pictures when "Speed" first crossed his desk. Even after the studio put it into turnaround, he still maintained an interest in making it his directorial debut because he had a wealth of ideas for how to ground the seemingly fantastical story.

For instance, his desire for a sort of workaday realism didn't stop with Bullock; it also extended to the characters on bus number 2525. It was important to him that the passengers reflect the multicultural world you encounter on the streets of Los Angeles every day. The studio pushed back, but de Bont was adamant that he have a large number of character actors along for the ride. He was interested not only in realism in his choices, but also the sort of intangible quality that comes with casting less-established actors and the honest reactions they can supply in the heat of an action thriller such as this.

"I really wanted people that happened to connect a little bit," he says. "You cannot 'act' out a lot of those scenes."

The very reason the film worked at all, Bullock says, is "because of every face and actor you saw on that bus. There was not one false casting note. You genuinely felt that these people would find each other on a bus, and their level of acting and showing the horror — I mean, we're driving in circles on a bus pretending. They were the ones that I really feel sold the premise and made the movie so good. It could have been so ridiculous, but instead it felt really real and gritty and fresh."

RELATED: 'Speed' 20th anniversary: Meet the passengers of bus 2525

Nevertheless, Graham Yost's original screenplay presented more opportunities for a number of those actors, opportunities that they lost as the project was developed further (right up until and during shooting, in fact). Joss Whedon was brought on very late in the game to do a re-write that streamlined the film. In some ways it was by artful necessity; many of the actors will tell you the script needed the tightening. But it was also an economic consideration, as de Bont recalls he wasn't able to give speaking parts to everyone who was expecting them due to the amount of residuals they would be owed in perpetuity.

"But I wanted to make sure there was a complete balance between the people that had some lines and the people that had no lines, because they're all the same characters, they all experience the same thing," de Bont says. "I told them, 'Listen, even if you think you're going to just sit on this bus, believe me, you're not, because I really want you to respond and relate to the events and I'm going to put you right in the middle of it. I'm not just going to try to hide you behind some speaking part. No, you're going to get a lot of close-ups. I need your reactions. I want the audience to get the same experience, that this is real; these people just happened to wait at the bus stop and get on and suddenly their lives are changed.'"

Whedon's lament

Yost — a showrunner these days for TV's "Justified" who sold "Speed" after years toiling away on series like "Full House" and Nickelodeon's "Hey Dude" — has readily admitted that "98.9 percent of the dialogue" from the film can be attributed to Joss Whedon. But the "Avengers" director, who was a well-regarded script doctor in those days patching up everything from Sam Raimi's "The Quick and the Dead" to the Kevin Costner bomb "Waterworld," was arbitrated out of credit for his work. Whedon spoke about his involvement in an interview with NATO's In Focus magazine in 2005.

"Part of what I did on 'Speed' was pare down what they had created, which was kind of artificial," he told journalist Jim Kozak at the time. "The whole thing about '[Jack Traven is] a maverick hotshot,' I was sort of like, 'Well, no, what if he’s not? He thinks a little bit laterally for a cop. What if he’s just the polite guy trying not to get anybody killed?'"

Whedon made significant alterations to the plot throughout as well, from killing off Jack's partner Harry (played by Jeff Daniels) to the disbursement of clues that would lead the LAPD to villainous former cop Howard Payne (Dennis Hopper) to transforming character actor Alan Ruck's role from that of a smarmy lawyer who gets dispatched to a gee-golly tourist who picked the wrong bus. But Yost — who Whedon has conceded is always very polite to him and is, again, quick to praise his contributions — was lobbied to push for sole credit and got it.

"At that time, and to this day, scripts are fluid," Reeves says. "I think the director has to put their stamp on it and actors come in. With Jan's vision, there was a kind of economy to it. There was still a lot of room. But I don't remember feeling any kind of, like, 'What's happening!? Where's the movie going!?' while we were doing it."

De Bont, who utilized Whedon's talents once again on his 1996 "Speed" follow-up "Twister," was also looking for, there again, authenticity in the rewrite. He felt the dialogue had to reflect how real people would more or less react in a situation like this, and that's no easy chore.

"They're not going to be long discussions on the bus," de Bont says. "It's all going to be quick and fast. And there's nothing worse and nothing more difficult or complicated than to come up with short lines for people in panic. It's one of the most difficult things you could ever ask a writer to do. We tried to come up with some believable variations and also sometimes let the actors on the bus see what they would do and what they would say, how they would react, because it had to feel real."

So he needed somebody who could think on his or her feet, someone who, if an actor couldn't come up with something, could spring into action. "I could call him early in the morning and say, 'Joss, I need two lines for this,'" de Bont explains. "And then he'd called me back 10 minutes later. He'd come up with some great little sayings that were basically continuing the tension, while at the same time pushing some relief into it as well, because you cannot have two hours of constant similarity in reactions. There are all these people who are turning a little cynical or trying to escape the danger by saying something lighthearted. He was extremely good at that and I really, really, totally have respect. I really tried hard to get him credit."

Additionally, there was an array of action beats that de Bont conceived, ideas that would come to him that he thought he'd like to see in a movie like this. That includes the iconic 50-foot jump the bus makes over a gap in the freeway, easily one of the key money shots of '90s action filmmaking.

The arbitration became a sticking point for Whedon. He's admitted that "Speed" is one of the few movies from that era that he worked on that he actually liked, but beyond one of the rare posters he owns that still bears his name, there's nothing to reflect his participation in the project.

"I’ve always just disagreed with the WGA’s policy that says you can write every line of dialogue for a movie – and they literally say this – and not deserve credit on it," Whedon told In Focus in 2005. "Because I think that makes no sense of any kind. Writers get very protective of themselves. They’re worried that some producer will want to add a line so he can put his name on it. But what they can do is throw writers at it forever without putting their names on it because of this rule. So I actually don’t think it works for writers. It certainly didn’t work for me."