A film school seminar on Smashed would probably examine the implications of the harsh natural lighting and deliberate lens flare in a stylised sequence with its two protagonists. It would speculate about such attempts to mimic the fragility and photosensitivity characterised by a hangover. And of course there would be other notions of what we think that writer-director James Ponsoldt might have meant, for example, by deliberately placing a red coat in the final scene. Luckily this isn't a seminar, as the movie goes beyond such trivial explorations.

Smashed follows the somewhat familiar lifestyle of recreational drinkers Kate (Mary Elizabeth Winstead), a primary school teacher, and her poorly motivated husband Charlie (Aaron Paul). It's entertaining. We watch them get drunk, do stupid things and of course have a great time. But these scenes are punctuated by often-subtle yet drastic instances of realisation that the drinking is a problem, which are foregrounded as the picture progresses. Anyone led by the advertising to expect a rom-com will be surprised, perhaps for the better.

"I don't know if it's the way it has been marketed in different places or just the way people have been perceiving it, but I've had both sides of the coin," says Mary Elizabeth Winstead. "I think either way people are going to be surprised. I think that's a good thing though, as long as they are willing to embrace it."

Smashed explores the fine line between recreational drinking and alcoholism, which feels timely, given that the differences between legal and illegal drugs are now being looked at in earnest. I wondered if Winstead felt a certain responsibility in portraying such a character and if she had to be careful about glamourising alcohol. "Absolutely, I was terrified of that," she explains. "I think what we wanted to do was show that it can be fun, that's the more realistic approach. Trying to make it seem like alcohol is bad all the time is gonna make it feel like a PSA. I was really, really in need of a challenge and I had never read anything that real and complex and funny and sad and just... so true to life."

Does that mean it's true to her own life experiences? "I haven't had experiences with alcohol that have scared me or even felt like I've had an unhealthy relationship with it in any way, so that wasn't really something I could relate to immediately."

But did it involve getting drunk at all? "Once," she says, without hesitation or regret. "Before we started shooting, Aaron Paul and I went out and got drunk. It was our first rehearsal and also our first time hanging out, so it was really good to become a bit closer in a very, very speedy way."

Rather than comedy or romance, the movie's essence lies in its realism. This is mostly down to an in-depth exploration of its characters: Kate is initially likable, but becomes a painful figure of desperation, humiliation and denial whom we feel sorry for but eventually no longer pity. Winstead remembers this as "quite an intense preparation process", the main body of which came from attending Alcoholics Anonymous meetings. "I just found so many ways to relate to them and to the process of recovery. If you take alcohol out of the equation... I had my own issues that I could use to tap into it."

Smashed also has Aaron Paul trade his usual on-screen vice of blue meth for Blue Curaçao (or whatever). The Breaking Bad star gives a powerful performance where expected, but is surprisingly even more convincing in the more sombre moments. It's thought-provoking to watch the progressive realisation of a couple who are in denial of the fact that their relationship was formed solely by drunken events. The narrative doesn't preach or assume too many stereotypes of alcoholism, but instead touches on these issues with subtlety. The real dangers are implied, but at times purposely understated.

Smashed isn't that visually interesting or preoccupied by aesthetic style, except for the aforementioned lens flare sequence, which feels quite forced within the context of such a natural journey. Maybe it is a hint at some genuine romance between Kate and Charlie, or perhaps just the work of a bored editor, but the way it sticks out proves that the film doesn't need to rely on adventurous visuals. This is a funny yet eye-opening depiction of a complex disorder, which is strongest when not drawing attention away from its compelling characters.

Smashed goes on nationwide theatrical release from Friday December 14.