The term 'Mechanic' refers to a professional hitman, who approaches his work methodically, like an architect or designer, and uses a very distinct process to research his target, develop a foolproof assassination method, and then coldly carry out the task.
The poster taps into this specialism with a clever visual device - a series of guns arranged to combine into a single revolver. The handle in particular is beautifully conceived and the overall effect is curiously amusing, despite the violent connotations.
The gradient-filled uppercase title is a hangover from the 1970's movie poster, and informs us that the film is basically a mainstream Hollywood action movie, rather than the arthouse European crime thriller that the design might initially suggest.
The poster was designed by award-winning design studio Ignition Print, and has a Saul Bass feel to it. It also reminds me of Exergian's recent TV serials posters.

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The Mechanic @ IMDB