I worked on Escape From Planet Earth at the end of 2012 and I was brought in to develop the background environment of this particular one sheet.  As you can see from the first image, the idea was much different with the aliens escaping from Area 51 out in the desert with casino-style buildings in the background.  The Weinstein Co also wanted the faces of the security guards to be seen.  I sketched out the background as well as the faces of the guards as you see below.  I also put some atmosphere in between the guards because they were getting bunched together too much and it was lacking depth.  They're running on hardened dirt at this point, so there has to be some environmental reaction to all of these things running around.
Further refinement as the right-side building was taken out.
They now wanted an exterior wall of the holding area to be visible behind the aliens and guards.
The Area 51 idea was scrapped and the larger buildings in the middleground were now back in place.  I also desaturated  the background with an adjustment layer just to focus more of the attention on the characters.  The ground plane was now switched out to asphalt since they're supposed to now be in a city setting.
Changing some of the far background buildings.
Added another middleground building to fill some space.
In-progress with rendering the buildings and characters.  The characters by the title are also temp at this point, but were left in there just so I knew where things were going to be placed.
The final, approved version that I sent off.  Of course the background saturation adjustment layer and the heads for the guards were in their own PS group that they could turn on or off.  I also gave them all the buildings in their own groups so that they could move the ones in the far background to wherever they thought worked better since at this point, they hadn't decided how large that crustacen-ish character was going to be.  The helicopters were also in temp locations.
This is the final one sheet that was used.  As you can see, everything is much more saturated and the characters by the title are different, guards have no visible heads and a bunch more subtle things that I had on layers(or somebody else tweaked) to turn on or off.