ALWAYS double-check points of stress -- doorknobs,
handles, hinges, stair-rails, bannisters, etc and
make certain these are SECURE. These things take
the most abuse night after night and will cause the
most danger or embarrassment should they fail.

Likewise, do a thorough sweep of the floor for any
loose nails, screws or errant hardware and debris.

As for concealing the void between flats:

FROM BEHIND SET: If the framing is made of thick
enough material, drill a hole in the framing of
adjoining flats and tighten these securely with a
sturdy carriage bolt, washers and wing nuts.
Alternate directions of the bolts and washers (one
from flat "A" to flat "B", the next from flat "B" to flat "A",
etc.) -- use as many as necessary to get the gap tight
and secure.

If the framing is not thick enough to accept a bolt,
then nail or screw 2x4 blocks at regular intervals
along the edges and drill through them instead.

IN FRONT: cover the void with that lovely 3M product
-- blue painter's tape. For added security you can
staple it to the flat's framing. Use the 2" width and
paint over it exactly as you did your flats. When
removed it leaves no residue.

Another solution is to add moulding to your set
design. Rather than attempt to camoflage the voids
between flats, a fluted moulding or raised panel
covers them in a decorative manner. Either paint
them the same color as your flat, or a complimentary
color to your design scheme. This works very well,
especially on Victorian interiors or sets requiring
older architecture.

You don't even need to nail the moulding on, but
merely lace a strong wire or cord through tiny holes
drilled in the moulding and secure it behind the flat
with cleats or bent-over nails.

When I am "building a room" to cover the seam between two flats I just use another strip of muslin  (4-6 inches wide) glued on,then painted the color of the flats. When you are done it will rip right off with out tearing the muslin underneath.  You can use masking tape too, but sometimes you can tell it is there.

I try to do some type of border around the top as well. Wallpaper or a wood trim work very nicley. It really "frames" the room up nicely. This only works well if you have a set you are not moving. I have used it on a set we "opened up" and it still worked well enough. It just was not a seamless as I would have liked.

Then I step back and look at it from the house and ask myself "what is missing?"  Sometimes it is just hanging a photo on the wall, or adding a vase of flowers to a table.

Best of Luck, hope this helps.

Aimee

What I always do is get some brown spraypaint and make "wear marks".  What I mean is add a light touch of paint around light swiches, door handles and any where I think my set would have been touched on a repeated basis.  I also go around the paintings hanging on the wall (be careful if these are borrowed, you may want to take them off the wall before you paint).

I then step back and look at the set as a whole and see if anything is missing - like swich plates, personal items that the people in story would want or need.  I also consult with the actors to check their interp of their character.  Details - Details - Details. 

Always check doors and doorknobs...

A play I saw once have the door fall of when the actors went through it...