"...Miloš Forman – being Czech played a role too."

It's almost certain he played a key role.

As background, I'll preface my comment by saying that I was once employed on the engineering side of the television industry, and the place where I worked was closely connected to film production, the complex included film labs that processed Eastman color stock for theater release and allied to it were film studios, so it's inevitable that I picked up some knowhow about what goes on during a film production.

Other than having seen Amadeus, (which I rather enjoyed but still hold several strong criticisms about despite its many Oscars), and the few snippets I've read about the film, I've no direct knowledge of its production per se, but I'd say that as its director Miloš Forman almost certainly would have had full oversight and a controlling say in all aspects of its production (as do most directors of any film).

Despite my background, I'm not really a strong film buff so I've no deep interest in the production minutiae of films but I took somewhat more interest in Amadeus than with most because of its story. I'm a long time enjoyer of classical music and I'm familiar with much of Mozart's and some of Salieri's music. I accept the modern view that their relationship was often more collaborative and there was little animosity or rivalry between them. In the relationship Mozart, if anyone, was likely the bigger problem, as he had a penchant thinking people were against him. There's some evidence that Salieri kept a watchful eye on Mozart, and as a fellow composer why wouldn't he do so as did Hayden and others? There's essentially no solid evidence that Salieri poisoned Mozart—only ill-founded rumors. In fact, evidence suggests he died of a combination one of the winter illnesses doing the rounds of Vienna at the time and a concoction of dangerous medicines (if Mozart was poisoned then the most likely culprit was his local apothecary).

Even before the idea of the film was born, it's almost certain that as a Czech Forman would have had a priori understandings about Prague that would have come to the fore when he learned he'd be directing the film. It's reasonable to assume he'd have immediately recognized that Prague would make an ideal readymade backdrop for the film. Its historical center was ideal from a production standpoint, it was of historical significance for the film and its old unaltered architecture could easily substitute for scenes in Vienna and Salzburg.

Moreover, I'd reckon it's highly likely (probably almost certain) that Forman was well aware of Mozart's close connection with Prague long before he'd read Peter Shaffer' stage play. It'd be difficult for him not to know, as Prague takes considerable pride that Don Giovanni, which is widely accepted as one of the greatest operas ever written and still a very popular† and still widely performed, had its premiere in the city in 1787 in the Bohemian National Theater (now the Estates Theatre). (I couldn't miss that fact when I first visited Prague as there was mention of it everywhere.)

"If you visit the cellar of an old house in Old Town / Lesser Town, chances are you'll find bricks much older than Maria Theresa."

Right. I've spent many an hour in such 'establishments' getting somewhat lightheaded (to put it politely) and I've made a practice of looking at their construction and becoming acquainted with their history and age. Where I come from (Australia) indigenous stuff is ancient but nothing of European influence is more than a bit over 200 years old so by comparison just about everywhere in Europe seems very old; in many places I've visited there's a surfeit of ancient buildings, they seem to be everywhere, we have nothing like that here. No doubt, to native inhabitants it's just the norm and they take it in their stride. However, for me the age and historical nature of these old buildings remains a novelty and they always pique my interest even though I've been to Europe many times and lived and worked there for periods.

"Whether or not they let you take a brick home is a different matter :)"

Ha, I doubt it. As an arch conservationist, it was never my thought to remove anything that was fixed in place, I recall once sitting on 2000-year-old bricks in the Colosseum and the thought never crossed my mind, what did however was that I was actually sitting on them and that no one else except me was the least bit interested in the matter.

Why would I remember that? Well, down here, we have social phenomenon—or at least we used to called the cultural cringe: https://en.m.wikipedia.org/wiki/Cultural_cringe. It's not as common these days as large sections of the community have come from Europe and elsewhere, and nowadays travel to Europe is commonplace, but it was alive and well when I was a kid and it rubbed off onto many of my generation (but it affected our parents and earlier generations to a much greater degree because few had the opportunity and means to travel). Like many of us, when I first went to Europe decades ago I thought I'd finally made it, my education was on its way to competition. Incidentally, sometimes it's still a sore point with older people who've never traveled.

The phenomenon is understandable in that before WWII the local culture here was profoundly British and the population was acutely aware that it was living in an outpost 12,000 miles from its cultural home of Mother England even though the vast majority had never been there. After the War that changed somewhat after several waves of European migration and later migration from Asia but native born Australians still knew they were missing out on experiencing the real thing—that of gaining actual hands-on knowledge of their cultural heritage and that it was important for them to make the pilgrimage.

Re: 'Maria Theresa' bricks, they had a particular resonance for me as I've long had an interest in the Enlightenment era and that I was living in one of the cultural capitals of the world. Also they weren't rare, there were many loose ones just lying about at various building and construction sites around Vienna. Moreover, I wasn't alone, a colleague, one of my fellow countrymen, would also hunt them down with gusto.

Clearly familiarity breeds indifference, such mundane historical artifacts like bricks seem to hold little interest for the locals but for us renegades from the cultural cringe they're considered prised finds. ;-)

On a similar theme I recall an occasion where I saw a nice old 18th Century single storey brick building in Kleine Neugasse near Wiedner Hauptstraße not far from where I was living at the time being torn down to make way for a modern structure. I was rather horrified at what I considered vandalism, which, to me, was all the more poignant due to the fact that nearby significant numbers of old buildings had been bomb-damaged during the War and out of necessity they'd been rapidly replaced with new ones of little architectural merit. I remember whingeing about it to my landlady over lunch at a nearby café. She wasn't the least concerned and couldn't understand why I was upset about it.

__

† Don Giovanni is also a favorite of mine, why wouldn't it be? It has appeal even for those who aren't opera buffs; it's salacious in parts and righteous in others, it's full of sex, promiscuity, seduction, rape, murder, revenge, arrogance, hubris, rejection of salvation and damnation—even a ghostly appearance from the murdered Commendatore! What else is there? The opera captures much of the human condition to a tee!

No wonder the conservative Viennese establishment banished it to Prague! Also, Don Giovanni tells us a lot about the liberal-minded rebellious attitudes of both Mozart and his librettist, Da Ponte. (BTW, Whenever we discuss Mozart's operas we usually leave Da Ponte out of the discussion. We shouldn't however, for without the Mozart-Da Ponte collaboration Don Giovanni would never have been the true and continuing success that it's been over the past few centuries).