Constellations and asterisms are generally not proximate in space, but rather happen to be nearby only when viewed from Earth. From Wikipedia's article on asterisms:
Like constellations, asterisms are in most cases composed of stars which, while they are visible in the same general direction, are not physically related, often being at significantly different distances from Earth.
A recognizable example of this would be the brightest star in Taurus, Aldebaran. Aldebaran appears in the V-shaped bull's head, mostly formed by stars in the Hyades star cluster. However, while those stars are actually close to one another, Aldebaran is less than half as far away.
Since the star is located (by chance) in the line of sight between the Earth and the Hyades, it has the appearance of being the brightest member of the more scattered Hyades open star cluster that makes up the bull's-head-shaped asterism; however, the star cluster is actually more than twice as far away, at about 150 light years.
Similarly, the stars of Orion's belt are actually hugely separated in space.
In addition, it's worth noting that the stars have different velocities across the sky, so the constellations will look increasingly different over long periods of time. Here is one prediction for 50,000 years time, and the effect will get proportionately larger as time goes on.
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