GLASGOW — Summer is generally the time for the blockbusters. It’s “Transformers 2,” packed with over two hours of giant robots smashing each other and little else. It’s time for the big kids’ movies like “Ice Age 3.” What we usually do not see is thoughtful character studies of historical figures and period pieces.

Director Michael Mann’s “Public Enemies” is both. The preview bills it as a shoot ‘em up gangster movie, and parts of it certainly are. It has all the trademarks: Criminals committing crimes, FBI agents pursuing them, shoot outs and car chases. However for most of the film’s two hour, 20 minute runtime we get part biography and part history lesson with small bits of action interspaced here and there.

A summer blockbuster it certainly isn’t.

The high points of the film are the performances by Johnny Depp as 1930’s bank robber John Dillinger and Marion Cotillard as his true-to-life love interest. The chemistry between the two is charming and one wishes there were many more scenes with the two of them together.

Christian Bale (The Dark Knight) gives his usual intense performance as FBI agent Melvim Purvis, tasked by J. Edgar Hoover to stop Dillinger’s crime spree across the Midwest.

Unfortunately, the performances are the strongest part of an overall weak film. While director Mann is clearly channeling his inner “Godfather,” “Public Enemies” lacks the deep connections and strong supporting cast that made that film an American classic. In fact, some characters are so unmemorable you lose track of who is who between scenes. Excellent actors like Billy Cruddup and Giovanni Ribisi are given little to work with.

While we experience Dillinger’s humanity and violence in equal measure, Bale’s FBI agent comes as little more than a starched shirt caricature of a Hoover era G-man.

The chase for the Dillinger gang is supposed to seem dramatic and convincing, it winds up rendered barely believable by the one dimensional characters surrounding Depp and Cotillard.

We loved The Godfather films for the way they drew the audience in, the way we connected with each individual role. Fredo might have been a secondary character, but his absence from the fishing boat was amazingly compelling. We loved “The Untouchables” because of the exceptional performances of Sean Connery, Kevin Costner and Andy Garcia. We cared about each and every character.

There certainly would have been room for that in the length of this film, but characters like “Babyface” Nelson and Frank Nitti are never fleshed out, or given more than passing attention. That lack of attention makes “Public Enemies” a very long 140 minutes and will prevent it from taking a place among the great crime dramas.

As a period piece, the film covers all the bases. The costuming is especially noteworthy, drawing on well-tailored men’s suits and clinging women’s fashions of the 1930’s. The city of Chicago plays a supporting role as, in the words of the Carl Sandburg poem: “Stormy, husky, brawling, City of the Big Shoulders.” It is wonderfully filmed by Mann who nevertheless should have spent far more time on the script than the lighting.

As a thoughtful period piece or a vehicle for two exceptional movie stars, “Public Enemies” is a passable success. But as a crime drama it ultimately falls under the weight of its own ambition. While the performances are certainly worth watching, it would have been better off released in September, when the kids are back in school.

A summer blockbuster it certainly is not.

Aaron Lane Morris is assistant manager at Marquee’s Highland 8 Cinema in Glasgow. He can be reached at aaronlane@gmail.com.