In Medias Res - perhaps the most overused dramatic device in modern fiction, it's impossible to throw a stone these days without hitting a movie that begins in the middle of the action. It's a daring move, or it would be, if most stories weren't using the weaker version of the device, in which, instead of starting in the middle of the story and trusting in the audience's ability to follow along, just teases the main action, then flashes back to explain how the characters arrived in the situation that starts the film.
While not completely inappropriate, this use of In Medias Res requires one thing - that the situation be unusual enough to require an explanation. If it doesn't, then all the film has done is wasted the audience's time. For examples of each type of flashback IMR, let's consider the films Thor and Skyline.
As Thor opens, a mysterious man has fallen through a wormhole and crashed into the ground. Who is he? How did he get here? This is pretty far from an everyday occurrence, so the next half hour of the film is spent detailing the amazing series of events that resulted in this absurd situation.
The film Skyline opens with the beginning of an alien attack - light beams down from the sky, and anyone foolish enough to look into it is drawn up to the spaceships far above. A few people staying in an apartment notice the lights, and one of them is captured. The the film then flashes back, not to explain the spaceships, but rather to let the audience know how a group of people wound up sleeping in an apartment. Since sleeping in an apartment at night is basically the least surprising thing that can happen, all the time spent establishing the series of events that led the characters to that point are a complete waste.
Also, apropos of nothing, you can't introduce a new kind of monster in the last minute of the movie, and expect us to just accept that's what the brains were for, Skyline.