Dans ce roman, les personnages sont très intéressants.

In this sentence, dans means “inside / in the pages of” the novel.

• dans is used for being inside something concrete or clearly delimited:
  
  • dans ce livre, dans cette boîte, dans la maison
• en is more abstract, often used with languages, months, materials, or some countries:
  
  • en français, en janvier, en bois, en France
    You don’t normally say en ce roman.
• à is rarely used with roman in this sense. À ce roman would sound wrong here; à is used in other structures (e.g. à ce moment, à Paris, à ce sujet).

So dans ce roman is the natural way to say “in this novel” in the sense of “within its story/pages.”

Ce is a demonstrative adjective meaning “this” or “that”. It must agree with the noun in gender and number:

• ce
  • masculine singular noun starting with a consonant:
    
    • ce roman, ce livre, ce garçon
• cet
  • masculine singular noun starting with a vowel or mute h:
    
    • cet homme, cet arbre, cet hôtel
• cette
  • feminine singular noun (any starting letter):
    
    • cette fille, cette maison, cette histoire
• ces
  • any plural noun (masculine or feminine):
    
    • ces romans, ces personnes, ces idées

Since roman is masculine singular and starts with r (a consonant), you must use ce roman.

Both can be correct, but they don’t say exactly the same thing.

• les personnages = “the characters”, all the characters in that novel (a definite, specific set).
  It suggests you’re talking about the whole cast:
  > Dans ce roman, les personnages sont très intéressants.
  > In this novel, the characters are very interesting (the ones in this novel).

• des personnages = “(some) characters”, not necessarily all of them, more indefinite:
  > Dans ce roman, des personnages sont très intéressants.
  > In this novel, some characters are very interesting.
  (This sounds like: only some of them are interesting, others maybe not.)

In your sentence, the idea is probably about the whole group of characters in the novel, so les personnages is the natural choice.

They all refer to people, but in different ways:

• personnage (plural: personnages)

  • A fictional or narrative character in a story, play, film, novel, etc.
  • Can also be a “character” in the sense of a colorful/odd person, but primarily literary.
  • Example: Les personnages de ce film sont très réalistes.

• personne (plural: personnes)

  • A real person or human being, neutral word, often formal or precise.
  • Example: Il y avait beaucoup de personnes à la réunion.

• gens (always plural)

  • Means “people” in a more general, collective sense, often informal and vague.
  • Example: Les gens sont très sympathiques ici.

In a novel, the imaginary “people” in the story are des personnages, not des personnes or des gens.

Adjectives in French must agree in gender (masculine/feminine) and number (singular/plural) with the noun they describe.

• Noun: les personnages → masculine plural
• Adjective: intéressant → must become masculine plural: intéressants

Other forms of intéressant:

• Masculine singular: intéressant (un personnage intéressant)
• Feminine singular: intéressante (une histoire intéressante)
• Masculine plural: intéressants (des films intéressants)
• Feminine plural: intéressantes (des idées intéressantes)

So if the noun changed, the adjective would change too:

• Ce roman est intéressant. (roman = masc. sg.)
• Cette histoire est intéressante. (histoire = fem. sg.)
• Ces histoires sont intéressantes. (histoires = fem. pl.)

In French, most adjectives normally come after the noun:

• un livre intéressant
• une voiture rouge
• un film drôle

Some common adjectives, especially short and frequent ones (often called BANGS: Beauty, Age, Number, Goodness, Size) usually come before the noun:

• Beauty: un beau film, une jolie maison
• Age: un vieux roman, un jeune homme
• Number: deux grands enfants
• Goodness: un bon livre, un mauvais film
• Size: un petit appartement, une grande ville

Intéressant normally goes after the noun.
You can sometimes put it before (les intéressants personnages), but that sounds very literary, emphatic, or old-fashioned, not neutral everyday French. The normal order is les personnages intéressants.

Each adverb has a specific use:

• très = very

  • Used to intensify adjectives and many adverbs:
    
    • très intéressant, très content, très vite

• beaucoup = a lot / much / many

  • Usually modifies verbs or nouns, not adjectives directly:
    
    • Il travaille beaucoup. (works a lot)
    • Il a beaucoup d’amis. (has many friends)
  • You don’t say beaucoup intéressants to mean “very interesting”.

• trop = too / too much / too many

  • Means excess:
    
    • trop intéressant = too interesting (implying a problem or excess)
    • trop cher = too expensive

Here you just want to say the characters are very interesting, so très intéressants is the correct choice.

Yes, you can also say:

• Les personnages sont très intéressants dans ce roman.

Both sentences are grammatically correct, but the emphasis is slightly different:

• Dans ce roman, les personnages sont très intéressants.
  → Starts with “In this novel”, so you emphasize this particular novel, then comment on its characters.

• Les personnages sont très intéressants dans ce roman.
  → Starts with “The characters”, so you emphasize the characters in general, then specify in this novel.

In everyday speech, both are fine; the difference is mainly about what you foreground first in the sentence.

Key points for pronunciation:

• dans → nasal an sound [dɑ̃]; final s is silent.
• ce → like “suh” [sə].
• roman → ro like “roh”, man with nasal an [ʁɔ.mɑ̃].
• les → like “lay” [le].
• personnages → roughly [pɛʁ.sɔ.naʒ]; final -es is silent.
• sont → nasal on [sɔ̃]; final t silent.
• très → “tray” but shorter [tʁɛ]; final s silent.
• intéressants → [ɛ̃.te.ʁe.sɑ̃]; final -s is silent, nasal an at the end.

Important liaison:

• les personnages → you should link: les‿personnages [le‿pɛʁ.sɔ.naʒ], with a z sound between les and personnages.

Spoken smoothly, it’s something like: [dɑ̃ sə ʁɔ.mɑ̃, le‿pɛʁ.sɔ.naʒ sɔ̃ tʁɛ ɛ̃.te.ʁe.sɑ̃].

• sont = present tense of être (“are”).

  • les personnages sont très intéressants →
    You’re making a general, timeless statement about the characters of the novel, as we see them now when we read the book.

• étaient = imperfect past tense of être (“were”).

  • Dans ce roman, les personnages étaient très intéressants. →
    Now you’re talking about them from a past point of view, for example:
    • telling someone what you thought of a book you read some time ago,
    • narrating in the past.

In practice:

• Use sont when you’re describing the novel in a general way (like a review, summary, or timeless comment).
• Use étaient when your narrative is anchored in the past (e.g. “When I was younger, I read this novel; the characters were very interesting”).