Barry Allen is not taken seriously by some of the "tough guy", hardass cops who see him as a science nerd. What does he know about the streets? Even if he is a vital part of their job working in the evidence room, the cops think he lives there, and it doesn't help his case that he is last to arrive and last to leave everyday, BUT he does good work, which is why he still has the job.
His first case in the lab is one that involves a murder with a mirror where he's being counted on to provide evidence to convict small time crook Sam Scudder (who we will get to in a moment), a trial is wanted ASAP for him and there's a short deadline. Barry ends up extracting the evidence to put Scudder away, and it is through this he meets Iris West, a reporter covering the story, who comes to him for questions. As it turns out they both met each other before but were never "officially" aquainted, the two "met" in college briefly in a rather embarrassing moment where Barry bumps into her on the way to a class he was late for a couple years back and drops his books, so Iris asks if she knows him from somewhere, there's a brief flashback on Barry's part "Nope, pretty sure we've never met", and then they proceed to talk business. There is a mutual, quiet attraction between the two, but that is not given much time to blossom thanks to hotshot Detective Russo, who sweeps Iris away right under Barry's nose and tries to set up a date to go out with her (or just down her pants, as Russo is a "macho" type and somewhat of a horndog). He tells Barry he'll need the lab results on his desk in the morning and to "pick up the pace". (another version of this is Barry and Iris are already dating prior to the accident, but the relationship is rocky and Iris is threatening breakup because Barry is undependable (i. e. always late), just a thought)
Anyway, because of the Scudder case being fast tracked to put him away, there is a ton of work that was put on hold temporarily in order to divert all efforts toward convicting Scudder that now needs to be done, Barry is commended for his work on Scudder by his boss, Captain Darryl Frye, but is disciplined for his tardiness/lateness, Frye warns him that there are people who will do his job AND be on time, and asks Barry to stay later that night and finish his work because of his late arrival and the heavy workload. It is this night Barry is hit by lightning and the accident that gives him super speed occurs (this is where fast because he is slow comes in, Barry Allen is the only major DC Comic characters to not only have his powers from an ACCIDENT but also because of his one, major FLAW; the man is always late). (Or they could give him the powers/accident prior to Scudder's case being solved and meeting Iris West, I just wrote it that way just to get the idea out there)
(^^ Would make a great poster for the film ^^)
Barry knows something has changed with him and that he should probably be dead, he uses his super speed in small doses at first and documents what's happening with himself as he's been taught with chemistry and criminology, plus being the slightly "methodic" person he is.
The first time Barry uses his powers to actually help someone it's to save Iris, who is reporting at a burning building in Central City where there are terrorists involved, a terrorist grabs Iris right in front of the camera and takes her hostage with a gun to her head. Meanwhile Barry, Russo, and Captain Frye, are watching. Russo and Frye take off, or maybe Frye is already there, but anyway Russo takes off to be the "big hero", and Barry is told he's too slow even if he were a "real" cop and to stay put. After they've left Barry slips out in his civilian clothes at super speed, he's never REALLY tested how fast he is yet, as he has only had his powers for 2 or 3 days. Anyway, his clothes burn off at the high speeds and anything left on him burns out in the fire (he's literally "The Flash", and he has not yet discovered his ability to vibrate his molecules at super speed which allows him to pass through solid objects). He has to keep moving for two reasons, so he isn't burnt and so people don't see him. Except for maybe running there, most of this scene is shot from the thugs point of view, Barry runs up the side of the burning building, dodges some bullets, and pulls Iris out and the few people left inside before it collapses. He then creates a suction of wind with his speed to smother the flames. People think he is an angel or a blur, some people have motion sickness and are not sure what happened but they are just happy to be alive. Barry escapes and thinks he's got to rethink the powers thing and next time maybe wear something that WON'T burn off him when he runs.
What the movie needs to establish is WHY he wears a red suit, without it taking up the entire film, as the audience already knows/expects him to be in the suit, get from point A to point B, but the in-between has to be good.
At first Barry has no intentions of being a superhero. He ends up wearing a super hero style "special" suit because (although not the complete suit yet) his ordinary clothes, which are loose fitting, burn and tear when he runs, they aren't protected by the aura around his body that shield him from friction (although he will later learn to expand his aura and vibrate so he can protect civillians when he runs with them). And the other reason is he likes comic books/superheroes, so the suit is sort of a fun thing to him. It's not that far fetched an idea, really. Look at our rock stars and things. People DO wear "eccentric/outlandish" outfits in real life, so I think this is somewhat plausible.
Maybe one day, Barry ends up stopping one random crime he notices, and it's not like doing this takes up his whole day or the most of his time now. He has ALL THE TIME IN THE WORLD as The Flash. All this escalates, one thing leads to another, next thing he knows he's now got a complete costume and is listening to a police scanner (which he builds into his ear-piece) and is somewhat of a hometown hero, a celebrity, the city has a fascination with him but the police would like to know who - or what, he is. Eventually Iris West will get the first tv interview with The Flash, which will be brief, but funny, and show what a down to earth guy he is.
Also, as The Flash, Barry is his own enemy in a way from a "business" perspective; Barry's day job revolves around him essentially picking up the pieces of crime's aftermath, being the "cleanup" guy in a way, as The Flash, now he is able to stop them before they happen IF he is fast enough. Like imagine a scene where Barry heads into work to find that there's nothing for him to do except try to piece together who The Flash is...and he just sort of sits there and scratches his head like "Well **** me..." . A scene like that would work well in the movie, I think.