I do not know to what an extent this term is used within film criticism or academia, if at all, but for me it might be the most important part of the film, even though it is difficult to capture and analyse. I have been reluctant to use it in my writings because it is sometimes seen as a vacuous word in itself, something used my ad-men to describe a restaurant or hotel lobby, but since the word and the meaning I attach to it matters to me, and not in a glib way but as something very real and fundamental, I have used it on two occasions this year, as a way of sneaking it into the conversation. First in the article about Death of a Cyclist (Juan Antonio Bardem 1955) and then in the long article about Ingmar Bergman in the 1940s. (Although the first time I used it was actually already in 2013 when I wrote about Krzysztof Kieslowski.) This is what I wrote concerning Bergman, about Thirst (1949): "In general the staging, pacing and ambiance just feels distinctly Bergmanesque for the first time."
With regards to film I use ambiance to describe the overall feeling the film gives me, but not to say that the film was sad, or cheerful, or angry, but the combined effect of every single detail of the film. How editing, lighting, sound, line-readings, costumes, shot-length, camera angles, blocking, visual clarity, camera movements, colours, grading, actor personality, and other aspects work together to create this ambiance. The ambiance is unique for each film and it is not in the script but always the surprising end-result of the filmmaking process, something that grows out of the cumulative decisions taken during the making of it, from casting and onwards. This is where the magic can happen, something never taught but which happen through chance. Of course, the filmmakers make decisions that are more or less informed, intelligent, sensible or sensitive (some will be better than others) but as there are so many imponderables and moving parts you can never be certain how it will all come together in the end.
There is something mysterious with all art forms, with our engagement with it. But not just with art, it is the same with people. We may say that we like a certain person because she is kind, or generous, or has a good sense of humour, but we do not like everybody with those qualities equally much. Some we simply like more than others, and some we deeply love, and why it is this person and not that person is beyond us to truly comprehend because we would have to go down on such infinitesimal details to separate them, even subconscious reactions. Like-wise, Edward Hopper's paintings are very similar in technique, brush strokes, colours, motifs and so on but there are some I like considerably more than others. Their difference in ambiance is the reason.
Ambiance works on many levels. For example, there is a certain ambiance to be found in French cinema in general, but it will also change over time, so that one film from the 1930s has a different ambiance from one from the 1960s, and there can of course be more than one at the same time too. But this ambiance does not just come out of the place in which it is made. Allan Dwan's 1932 film While Paris Sleeps is set in France but it is not shot there and the dialogue is in English, as it is an American production. Yet its ambiance is strikingly French and not necessarily that of a typical Hollywood production of 1932. The same year Dwan made a film in Britain, Her First Affaire, and this has all the ambiance of a British production. So here the film is inflicted with the ambiance of its place of making whereas in While Paris Sleeps the ambiance comes from its setting, and probably from Dwan having watched French films for inspiration. Dwan is a highly gifted filmmaker but as of yet I have not felt a distinctly Dwanian ambiance, unless a chameleon-like (or Zelig-like) ambiance is his own unique quality. Although perhaps his later films like Silver Lode (1954), Tennessee's Partner (1955) and Slightly Scarlet (1956) exemplify a more consistent ambiance. (Now that is a great trio which is far from getting its due.)
While Paris Sleeps
But in general most filmmakers have their own ambiance, which could be seen as the one particular thing that really separate one from the other. There is in general talk of style and theme, but they can often overlap between filmmakers and many times they can be more in line with studios, genres, eras or movements. But the ambiance is different. Henry Hathaway and Henry King were both long-time employees at Twentieth Century-Fox under Darryl F. Zanuck and their films overlap in many ways. But they feel different. King is religious and Hathaway is not, which is partly a reason. King has in general a more opulent mise-en-scéne, stronger colours and more elaborate lighting patterns, and Hathaway has a more stringent and less gaudy style, and this also contribute to this difference. But it is also a question of movement, blocking, gesture, acting, attitude, humour and emphasis, all those things that are often not in the script but directorial decision made on the spot, from instinct as well as from personal theories about how it should be done. And every director will have their own ideas of how it should be done and therefore all directors are different, just as all other contributors to a film have their own ideas about how their particular contribution should be done. This is one reason why it is so interesting and enlightening to compare two filmmakers as superficially alike as Hathaway and King. Anybody can see that Claire Denis and Nora Ephron are different so there is not much excitement to be had from talking about that (as opposed to discussing their similarities), just as it is obvious how a film by Chantal Akerman is very different from a generic Hollywood film. But so what?
I said in my article about Ingmar Bergman's films of the 1940s that they felt different from one another. How his first film, Crisis (1946), felt closer to an American melodrama whereas his second film, It Rains on Our Love (1946), felt much closer to a generic Swedish film (with a touch of French poetic realism) and this is also a question of ambiance. It is partly about non-specificity, i.e. there is not much that you would have had to change if the script for Crisis were to have been shot by Universal in Hollywood instead of SF in Råsunda, but also partly about the less tangible things which are hard to describe but are just there, which is what ambiance is.
It can also change within films. To take a recent example, the opening sequence of Solo: A Star Wars Story (Ron Howard et. al. 2018) was really bad. It was tonally off, the dialogue did not work and there was no chemistry. But after the opening was over (where young Solo and the female lead Qi'ra are introduced) and the main story begins, after a sign saying "Three years later", it was like a completely different film. The ambiance had changed from one cut to the next, and what had been a bad film turned into a reasonably good film. I do not know if this was because of the chaotic production or what happened there, but it was interesting.
But ambiance is difficult because it takes a lot of hard work and effort to locate it and recognise it, and you have to watch a lot of films. This is not something that you can learn by readings books, it can only be picked up by watching films. If I watch one film from a country from where I have seen no other films, by a director of whom I have seen no other films, I can tell whether I like it or not but I have no idea where its ambiance comes from. I will be at a disadvantage. (This is something I find annoying when people write about Bergman's films of the 1940s and 1950s without having seen any other Swedish films of that time, yet do not take this fact into consideration in their analysis.) Yet, as I said above, ambiance is a central aspect of any film, and of any filmmaker's oeuvre; it should be at the heart of any auteur study.
Through the Olive Trees (Abbas Kiarostami 1994)
But there is no right answer as to what a given film's ambiance is. That is part of the process of engaging with art (of whatever kind). I prefer to see our experience of, or engagement with, art as a process, or dialogue, between us and the art work, which tends to be unstable in what meaning we take from it. Ambiance is part of that meaning, might even be its meaning as art, distinct from whatever political or philosophical meaning we might see in the work. So I will just end this essay by suggesting that it is in a film's ambiance that the art of the medium is to be found. The ambiance is the art.
Avant-garde films or installations are often pure ambiance, as they have no story, narration or actors.