Learn Hardcore Portuguese (Portugal): O autor descreve cada cenário com tantos detalhes que quase vemos o filme na cabeça. - The author describes each setting with so many details that we almost see the movie in our head.

O autor = “the author” (a specific author already known from context: the author of this book, this article, etc.).
Um autor = “an author” (any author, not a specific one).

In book reviews, summaries, and commentary, it’s very normal in European Portuguese to say O autor… because speaker and listener usually know which author is meant from the context.

What exactly does “cenário” mean here, and is it masculine or feminine?

Cenário is masculine: o cenário / os cenários.

Typical meanings:

literal: the physical setting / background / scenery of a scene (in a play, film, novel).
more abstract: a scenario or situation (possible future, hypothetical situation).

In your sentence, cada cenário means each setting / scene / situation the author describes in the book. Because it is masculine, other words agreeing with it would be masculine (e.g. este cenário, um cenário bonito, etc.), but here only the article o in the singular form appears (dentro de cada cenário is not there, just cada cenário).

Why is it “com tantos detalhes” and not “em tantos detalhes” or something else?

Portuguese usually uses com to talk about the manner in which something is done:

com muitos detalhes = with a lot of detail
com cuidado = carefully / with care
com paciência = patiently / with patience

So descrever algo com detalhes is the natural collocation: to describe something with detail.

Em detalhes also exists (e.g. explicar algo em detalhes = explain in detail), but in this sentence, com tantos detalhes is the most idiomatic and common choice.

What is the function of “tantos” in “com tantos detalhes”, and how is it different from “tão”?

Tanto / tanta / tantos / tantas is a quantity word (“so much / so many / such a lot of”), and it must agree with the noun it modifies:

tanto dinheiro (masc. sing.) – so much money
tanta água (fem. sing.) – so much water
tantos detalhes (masc. pl.) – so many details
tantas pessoas (fem. pl.) – so many people

In contrast, tão is used mainly with adjectives and adverbs, not directly with countable nouns:

tão bonito – so beautiful
tão rapidamente – so quickly

You can see the structure clearly in your sentence:

com tantos detalhes que… = with so many details that…

This is a typical result construction: tanto(s)… que… (“so much/many … that …”).

What is the role of “que” in “com tantos detalhes que quase vemos…”?

Here que introduces a result clause:

com tantos detalhes que quase vemos…
= with so many details *that we almost see…*

The structure is:

tanto(s) / tanta(s) … que …
tão … que …

Both express degree → result:

Ele fala tão depressa que não percebo nada.
He speaks so fast that I don’t understand anything.

So in this sentence, que is not “because” (that would be porque), but the linking word for “so … that …”.

Why is the verb “vemos” (we see) and not something like “vê-se” or “vês”?

The subject of the second clause is we (nós), not the author:

1st clause: O autor descreve… – The author describes…
2nd clause: (Nós) quase vemos… – (We) almost see…

Portuguese often drops the pronoun because the verb ending shows the person:

vemos = we see (1st person plural).

Using vê-se would change the meaning to something more impersonal:

vê-se o filme na cabeça ≈ one sees / you see the film in your head (more generic, not explicitly “we”).

The writer chose vemos to involve the readers directly: we almost see the film.

Why is everything in the present tense (descreve, vemos) instead of a past tense?

Portuguese, like English, often uses the “narrative / descriptive present” to talk about books, films, and stories in a lively way:

Neste livro, o autor descreve… – In this book, the author describes…
No filme, o protagonista morre no fim. – In the film, the protagonist dies at the end.

So:

O autor descreve… = the author (in this book) describes…
quase vemos… = we almost see…

Using present tense makes the description feel more immediate and timeless, even if the book was written long ago.

Past tense would also be possible in some contexts:

O autor descreveu cada cenário… (the author described each scenario…)

…but the neutral, “review-style” choice is the present.

What does “na cabeça” really mean here? Is it always “in the head”?

Literally, na cabeça = “in the head” (em + a = na).

In this context, it’s a very natural idiom meaning “in our mind / in our imagination / in our head”:

vemos o filme na cabeça ≈ we see the film in our mind’s eye.

In European Portuguese, it’s quite normal to omit the possessive here:

na cabeça (literally “in the head”) = contextually understood as na nossa cabeça (“in our head”).

You could say na nossa cabeça to be explicit, but it’s not necessary and na cabeça sounds very natural and idiomatic in this sentence.

Could you also say “quase que vemos o filme na cabeça”? How is that different from “quase vemos”?

Yes, you could say:

…com tantos detalhes que quase que vemos o filme na cabeça.

In everyday European Portuguese, quase vemos and quase que vemos are both used.

quase vemos: a bit more neutral / standard.
quase que vemos: slightly more colloquial and emphatic; it can sound a bit more spoken or expressive, depending on the region and tone.

Meaning-wise, they are extremely close: in both cases, we almost see the film in our head.

Could we say “vemos quase o filme na cabeça”, putting “quase” after the verb?

That sounds unnatural in Portuguese. The usual positions for quase in this kind of sentence are:

quase vemos o filme
quase que vemos o filme

Placing quase after the verb (vemos quase o filme) is usually only natural when quase modifies a quantity or something different from the whole action, e.g.:

Comi quase a maçã toda. – I ate almost the whole apple.

Here, we’re modifying the action “see the film” as a whole, so quase goes before the verb (or in quase que + verb structure), not between the verb and its object.

Is there any difference between “cada cenário” and “todos os cenários” here?

Both are possible but have a slightly different nuance:

cada cenário = each scenario / every scenario
- focuses on individuality: the care and attention to each separate scenario.
todos os cenários = all the scenarios
- focuses on totality: the whole set of scenarios.

In this context, cada cenário subtly emphasizes that every single scene is described in rich detail, one by one.

Could we say “com muito detalhe” instead of “com tantos detalhes”?

Yes, you could, but it changes the nuance:

com muito detalhe = with a lot of detail (a high amount of detail in general).
com muitos detalhes / com tantos detalhes = with many details / with so many details, suggesting numerous little elements, pieces of description.

Your original sentence:

com tantos detalhes
- que quase vemos…
  stresses that there are so many separate, vivid details that it’s as if we’re watching a film in our head.

So com muito detalhe is acceptable, but com tantos detalhes supports better the idea of “so many tiny points that the scene comes alive”.