Got around to seeing American Hustle tonight, and liked it better than most new movies I see. One thing took me out of the moment several times, though -- the improv scenes.
In a period movie, actors should stick close to the script because they are unlikely to be able to improvise and maintain the period's authenticity on the fly. When the acting is more spur-of-the-moment, it will come from the actor's gut, which is tuned to the here-and-now.
There was a scene where Irving Rosenfeld asks "Really?" in a slightly flat and miffed voice, in response to someone else's ridiculous behavior. That's too specifically from the 2010s. At the end when Richie DiMaso does a mock performance of his namby-pamby boss, it's so over-the-top and laughing-at rather than laughing-with, that it felt more like a frat pack movie from the 21st century. Ditto the scene where DiMaso is trying to convince Edith to finally have sex, where the dialog sounds like it's from a doofus rom-com by Judd Apatow.
These and other scenes should have been in the outtakes -- wackiness ensues when the actors break character! When they're left in, it creates jarring shifts in tone, as though an actor who'd been speaking with an English accent switched to Noo Yawka for half a minute, then switched back to English (all for no apparent reason).
Improvising and wandering slightly outside of the character's range isn't that jarring. Like, maybe you're just seeing a slightly different and unexpected side of them in this scene. But anachronism is not so easy to suspend disbelief about -- it definitely does not belong to that period. If they turn on the radio in 1980 and it's playing Rihanna, that kills the verisimilitude.
It's even more baffling in American Hustle, where the costume, make-up, and production design has been so meticulously worked to make you believe you're looking in on a certain time and place. All it takes is a series of distinctively 21st-century improvs to throw that into doubt for the viewer.