DC comics did a good job of keeping things separate between the DCEU and the Arrowverse. There were heroes that DC wouldn’t allow in the Arrowverse, and vice versa. There were a few that did appear in film and TV, with Superman showing up in both versions, but The Flash took it one step further. The two Flash’s actually appeared together in the Arrowverse.
Ezra Miller portrayed Flash in the DCEU, starting in Justice League, and unsurprisingly, he's dramatically different from Grant Gustin's Arrowverse Flash. One is a police forensic officer who has a grip on his life, and the other is an insecure young hero who wasn’t sure where his life was heading. But which was more accurate to the comics?
Ezra Miller - Genius
In the comics, Barry Allen is an intelligent hero. In DC Comics, Allen earned the role of the Supreme Chemist in history, creating his own Philosopher’s Stone. Barry also has a genius-level intellect in chemistry and forensic sciences.
While The Arrowverse’s Flash is smart, the DCEU version is a genius. When Bruce Wayne found his home, there were several items that Barry created for his career as Flash. In the Arrowverse, Flash relied on his friends to create everything.
Grant Gustin - Forensic Scientist
Barry Allen is a brilliant forensic scientist in DC Comics and has a genius-level knowledge of chemistry to help him solve crimes for the police department.
In the DCEU, Barry Allen got started on his journey to this point, but in the Arrowverse, Barry was already there. From the first episode, Flash discovered clues to help the police solve crimes and proved to be both a fantastic forensic scientist and a great hero, just like the DC Comics version.
Ezra Miller - Insecure
The Flash was one of the first superheroes DC Comics introduced in the Silver Age. He is a police forensic scientist who helps solve crimes, so it comes as no surprise that he has been best friends with Bruce Wayne, and Batman respects him. Despite all this, Flash has also been one of the most insecure heroes in DC, and that is Ezra Miller’s version.
Barry has caused multiple disasters because he never overcame the loss of his mother when he was a child. Flash is at his best when around someone stronger-willed, and that certainly applies to the film version.
Grant Gustin - Rash Decision Maker
In DC Comics, Barry Allen’s worst action was running back in time to stop his mother’s murder. What resulted was Flashpoint, and it took all Flash could do to help Thomas Wayne’s Batman put the world back together again. The same thing happened in the Arrowverse.
Grant Gustin’s Flash has always acted before thinking, and it often causes problems. For someone who works with the police, Barry uses his instincts more than his brains, and it causes as many problems as it fixes.
Ezra Miller - His Passion
One thing that makes Flash shine in the DCEU is his passion for being a hero. Flash there plays closer to the comic books than the Arrowverse does, as Grant Gustin goes about business as a hero but Ezra Miller seems to want to be a hero, working well within the confines of the Justice League.
Even when injured, Ezra’s Flash refuses to give up and shows the grit that DC Comics fans love from The Flash.
Grant Gustin - Always Late
One of the funniest things about The Flash in DC Comics is that the Fastest Man Alive is always late. There is something to be said about a man who can time travel but can’t get to an appointment on time to save his life.
The Arrowverse has played into that trait throughout Grant Gustin’s run on The CW series. In the pilot, Barry was late getting to the crime scene to gather evidence, and he has remained unreliable ever since.
Ezra Miller - Showing His Speed
The Arrowverse and DCEU show Flash’s super-speed in different ways. In The Flash, Gustin’s Barry Allen runs like a blur through the town, his speed so blazing fast that only a streak remains. While that is a good way to show it on the small screen, it isn’t the best way to show how fast Flash is.
The DCEU does him justice in the opposite manner than Fox did with Quicksilver in its X-Men movies. They show Ezra’s Flash and then cut to someone else and he has already done something. He isn’t so fast that he is a streak; he is so fast that no one, not even the audience, sees him move.
Grant Gustin - His Costume
When The Flash first appeared on The CW, his costume wasn’t the most comic book accurate. It was more in line with what Fox did with the X-Men, but as the MCU has shown comic-accurate costumes can still look cool on the movie screen, each season of The Flash has added a little more to make it a comics-accurate portrayal.
Ezra’s costume in the DCEU is nothing like the comics, but the Arrowverse tries its hardest to remain loyal.
Ezra Miller - Wally West
There are several people who took on the role of Flash in the comics. However, the two most popular were Barry Allen and Wally West. After Barry died in Crisis on Infinite Earths, Wally took over the role after serving as Kid Flash.
While Ezra Miller is Barry Allen in the DCEU, he is more comic’s faithful to Wally West’s version, a young hero who always wants to fit in and tries to learn at the feet of the iconic Justice League members.
Grant Gustin - Barry Allen
While Ezra Miller is more Wally West than Barry Allen, Grant Gustin’s Flash is the Allen version. From his work as a forensic scientist to his role in mentoring younger heroes in his world, Gustin is everything Allen was in the comics.
Gustin also has Flash's love relationship with Iris West and the death of his mother that he always tries to make up for driving him forward. Miller could get there, but in the Arrowverse, Gustin already fits the character like a glove.