A Flash film is inevitable, should DC go through with they're planned cinematic universe. I've compiled a list of actors who I think would do well in a movie about the fastest man alive. Bear with me, however, as this is my first fancast.
Dominic Monaghan as Wally West/The Flash
I know, its a stretch. But I think he could do an excellent Flash. He looks somewhat like Wally, and is the perfect size. He also can bring forth the perfect combination of laid back charm, and seriousness. I might be wrong, and it is quite possible, but I've been yearning to see Dominic Monaghan in a superhero role.
Yunjin Kim as Linda Park
I'm probably going overboard with these Lost actors, but I think she'd be good for the role.
Benjamin McKenzie as Barry Allen/2nd Flash
When I see him, I see Barry Allen for some reason. This movie would likely take place after the events of Final Crisis, so his role would probably be limited to flashbacks (no pun intended).
Alexis Denisof as Sam Scudder/Mirror Master
He voiced Scudder in JL: DOOM, hes good at playing the villain, and could be great as the live action version to kick off what could be a great trilogy.
Jackie Earle Haley as Captain Cold
The man is good at being sinister. I think he show the world that Captain Cold is no Mr. Freeze ripoff.
Jake Gyllenhaal as Eobard Thawne/Professor Zoom
Another big stretch, but I think it could work. The second Zoom would be hard to bring in, without Grodd and the cosmic treadmill. Gyllenhaal is great actor who could bring a lot to Professor Zoom.
As I stated before, a lot of it might seem unreasonable. But Chris Nolan took as many artistic liberties as he wanted with his Batman movies, and I think Shawn Levy could do the same with Flash.