(Although I tried to refrain from spoilers, this review might include some.)
If somebody asked around what does the films Se7en and The Shawshank Redemption have in common, chances are that most people would say that both star Morgan Freeman. But these two seemingly different films have a lot more in common, other than their lead actor.
The first and simplest similarity is their release date. Both Shawshank and Se7en were released within a year of each other. The former was released in October 1994, while the latter was released less than a year later (September 1995).
A second similarity might be each film's position within their directors oeuvre. Both films could be considered as their respective director's "first" full-length feature film. Sure, David Fincher was hired to direct Alien3 two years before Se7en, but the production of that film was so troubled that the filmmaker himself refuses to acknowledge it as one of his films, and doesn't even include it in his filmography. Fincher had severe disagreements with the studio, and he abandoned the project during post-production. With Se7en, he was allowed to have more control over the final product, and thus it could be considered HIS "first" film. Frank Darabont, on the other hand, had a successful career as a screenwriter. But his only films before Shawshank were some shorts and a TV-film. The prison drama was his first full-length feature film.
The third similarity, as mentioned above, is the most notable: the cast. Both films star veteran actor Morgan Freeman, paired with a younger actor (Tim Robbins on Shawshank and Brad Pitt on Se7en). In the first film, Freeman plays Red, a longtime convict who befriends Robbins' character: a banker wrongly accused of the murder of his wife and her lover. On the second film, Freeman plays William Somerset, a veteran detective who is days from his retirement. During his last week, he is paired with his replacement, a young detective played by Pitt.
But the similarities doesn't end there. The constitution of these pairings are also fairly similar. In both films, Freeman plays a fairly pessimistic man, defeated by the odds around him. Be it life in prison or accepting the violent city around him. Both characters have given up on their fights for freedom and hope, or against crime, and have surrendered to their inevitable fate. In Shawshank, Red is an institutionalized convict who has accepted his role as the "go to" guy for prison contraband; while on Se7en, he's just cruising towards retirement, longing for a life outside of the city, away from the crime he feels he was unable to fight against.
On the other hand, both of Freeman's companions have a different outlook on life than him. In Shawshank, Andy seems to be unfazed by the circumstances around him. He manages to make prison life different for his friends, while always holding out hope for the future. This tends to rub the hopeless Red the wrong way, despite their good friendship. In Se7en, Mills arrives to this nameless, violent city, expecting to be the "hero" to make a difference. His energetic persona and idealistic view of life also rubs the old and weary Somerset the wrong way, which makes them butt heads at first.
In both films, Freeman's character expresses what his future will be to his new, younger companion. In Shawshank, he considers himself "institutionalized", and when Andy asks him if he thinks he'll ever get out, he replies: "Sure. When I got a long white beard and about three marbles left rolling around upstairs." In Se7en, he considers his job pointless. And when Mills confronts him, and asks him what he thinks they're doing, he coldly replies:
"Picking up the pieces. We're collecting all the evidence, taking all the pictures and samples. Writing everything down, noting the time things happened... That's all. Putting everything in a neat little pile and filing it away, on the off chance that it will ever be needed in a courtroom. Picking up diamonds on a deserted island, saving them in case we get rescued."
Both Red and Somerset are different representations of defeat, despair, and hopelessness. Red has pushed away his dreams of freedom and seeing the Pacific Ocean (which are more notable in the short story than in the film), and has settled into his comfort zone, waiting for his life to wilt away. Somerset seems to be wandering through his job, awaiting the moment to retire, to walk away from all the evilness around him, and to safely retreat away from it all, where he can watch things from afar, a victim of the own apathy he criticizes.
Finally, both films have spectacular final acts. Starting with the moment where Andy doesn't walk out of his cell, or when John Doe walks into the police station, both films start intense sequences that challenge our characters outlook on life. From despair to hope, or viceversa. In Shawshank, Red's hopelessness and conformity is challenged by Andy's ability to break away from the mold, and forces him to change the direction he had assumed into one of hope. In Se7en, it is Mills' idealism which hits a wall, as his character ends up consumed by the "evil" around him. Those two film sequences are among the best I've seen on any film.
A final similarity, more personal than anything, is that both films have remained as my favorites for more than 15 years. Both films struck me in different ways: Shawshank in an emotional, intimate level, and Se7en in a more visceral one. Both films changed the way I saw films and still manage to thrill me and amaze me, whenever I rewatch them. I'm pretty sure I've seen "better" films, but I don't think any other has had the effect these two had in me.