For Manchester scenes (filmed on actual streets in the USA), any overly "American" curbside items (like certain fire hydrants) were hidden by dropping bottom-less slatted metal trash cans over them and then adding prop "English" rubbish; however, extras and crew unaware of this subtle artful touch continuously filled the apparently-normal-looking receptacles with their own trash. Between filming sessions, rueful set dressers would have to remove a foot-high layer of discarded plastic water bottles (and then reset and fluff the "official" rubbish).
The plot is set in various international locations: Manchester (United Kingdom), Samarra and Balad (Iraq), Qatar's US Military Base, Amman (Jordan), Amsterdam (Netherlands), Langley and Washington DC (United States), Dubai (United Arab Emirates), Incirlik (Turkey), Vienna (Austria) and Dar'a (Syria) but the actual filming locations were only Morocco and United States.
Some performers in the exterior Turkish Cafe in Munich Scene were stage-reading actual Arabic newspapers. The more-attentive among them were indeed reading from right to left (most noticeable when they turn a page).
For Manchester Scenes (filmed in a gritty and real American slum), actual English "police line" tape was used. Its distinctive blue and white coloration contrasts with US-style police tape which is yellow with black text. Comparison was easy because occasional stray bits of American police line tape from past actual crime scenes were among the real neighborhood's windblown street debris littering the edges of the filming area.
To extras waiting to join the "Terrible Neighborhood" and "Terrible Flat" scenes, a wardrobe specialist announced, "Please remove your personal jewelry including school rings and wedding bands; this is supposed to be a gritty neighborhood full of crackhouses and you're poor... you hocked all of your jewelry - for drugs."
During one Munich scene (actually filmed on a busy urban US street corner), civilian vehicle traffic was stopped only during actual filming. Just before and after filming, the prop street signs (written in German) were in place while the street was still open to traffic. Thus, some unknowing motorists went from seeing typical street signs (which said things like "Central Avenue" or "Washington Street") to reading differently-colored German signs for, say, "Charlottenstraße".
Costume standards were such that performers portraying British police wore full regalia including a tie, utility belt, regimental insignia, and a bulletproof jacket even though none of these were visible under their blaze yellow police emergency-response raincoats. Also, these same police were usually positioned with their backs to the camera - which was often a quarter-mile away.
When Ferris first meets with Hani, he is asked about what he has heard about Hani's unit of intel gathering and interrogation methods. He responds by saying "finger-nail factory". Mark Strong was in Syriana (2005) and tortured George Clooney's character by pulling out his finger-nails.
Carice van Houten actually played the character of Roger's wife Gretchen Ferris, but all her scenes were deleted and she does not appear in the final cut. The movie does provide dialogue that distills the story in the book, when Roger says they are getting divorced because "I was a bad husband and she was a worse wife".
The Amsterdam scene in the picture was actually shot at Eastern Market, a historic marketplace in Southeast Washington, DC, which hosts weekend food and flea market vendors. It was shot on a hot September day, though extras had to suffer wearing heavy clothing for what was supposed to be a cool fall day. The car explosions were really staged - not special effects.
During filming in the USA, a few of the POVs (privately-owned vehicles) which extras used to reach the locations were used during filming of scenes meant to portray parts of Europe, but first the cars needed to be "de-Americanized". This involved more than just switching license plates. Rear-view mirror dangling trinkets (graduation tassels, novelty-shaped air fresheners, etc) were removed. Bumper stickers advertising vacation spots and political views were scraped off. In some cases, windshield decals from European municipalities were added.
To get just the right look during a massive pyrotechnic explosion while also not damaging still-occupied neighborhood buildings, technicians used a "car chucker" (rather than the force of the blast) to fling a vehicle violently. The fridge-door-sized steel plate has a powerful spring-hinge device able to catapult cars for specified distances.
A visual sequence meant to illustrate cross-cultural suspicion by showing German police and Turkish migrant-workers eying each other darkly at a Munich street corner's outdoor tea café was cut. A short clip of the sequence survives, but as a sort of Islamic "anytown", without the German-language street sign or the Munich police car. It appears as an aside In Scene 2 while Hoffman briefs headquarters staff about "men from the future", Americans, being easy to defeat when terrorists use face-to-face meetings and hand-written notes in place of easily-traceable electronic technology.
Over the course of filming the Terrible Neighborhood scene, two tanker-truckloads of water were sprayed onto the location neighborhood streets because, for looks, wet roadways were considered preferable to dry pavement.
While the derelict-but-surviving US neighborhood where the Manchester Scenes were filmed had plenty of its own street litter and urban debris, the Hollywood crew (in the effort to make the area look like an English slum) had left certain piles of prop rubbish in precise places. For scene continuity, such rubbish needed to remain present - even when scenes took multiple days to film. To protect against unwitting community litter cleanup, "essential" debris was flagged overnight and on weekends with "hot set" tape (a specialized version of other American yellow hazard tapes which say things like "caution caution caution", "wet paint", or "police line do not cross").
The trivia item below may give away important plot points.
During Scene 1 which included the "Terrible Neighborhood" sequence, set dressers pasted street posters for real musical bands upon walls and lampposts. While the selection of posters was generally random, there was careful attention paid to the posters seen just before the tactical squad storms the bomb-makers' flat: they are bright red with the headline SMOKE OR FIRE ABOVE THE CITY. The red sky and black cityscape purposefully foreshadows the massive explosion and fireball (indeed, "smoke or fire above the city") which is seconds away.