Hlavní postava má strach.

Because mít strach is a fixed and very common Czech expression meaning to be afraid / to feel fear.

Literally, it is to have fear:

• má = has
• strach = fear

So Czech often uses mít where English uses be:

• Má hlad. = He/She is hungry. (literally: has hunger)
• Má žízeň. = He/She is thirsty.
• Má strach. = He/She is afraid.

Using je strach would not work here.

It is in the nominative, because it is the subject of the sentence — the person/thing doing or experiencing the action/state.

Here:

• hlavní = main
• postava = character

So hlavní postava is the subject: the main character.

Also, postava is a feminine singular noun, so the whole subject phrase is feminine singular.

Strach is in the accusative singular, because it is the object of má in the expression mít strach.

A useful detail: for this noun, the nominative and accusative forms look the same:

• nominative: strach
• accusative: strach

So even though the form does not change, its grammatical role does.

Because hlavní belongs to a type of adjective whose nominative singular form is -í for all genders.

So in the nominative singular, you get:

• hlavní muž = main man
• hlavní postava = main character
• hlavní město = capital city / main city

Unlike adjectives such as dobrý / dobrá / dobré, hlavní does not change in the nominative singular according to gender.

However, it does decline in other cases:

• bez hlavní postavy = without the main character
• k hlavní postavě = toward the main character

Because Czech has no articles.

English distinguishes:

• a main character
• the main character

Czech usually leaves that idea to context, word order, or other words if needed.

So Hlavní postava má strach. can mean:

• The main character is afraid
• or sometimes A main character is afraid

In most contexts, hlavní postava will naturally be understood as the main character.

A rough English-friendly guide is:

HLAHV-nee POS-tah-vah maa strakh

A few details:

• hl at the start of hlavní is really pronounced; don’t drop the h
• á is a long vowel, so má is longer than ma
• ch in strach is the Czech ch sound, like the ch in German Bach or Scottish loch, not like English ch in church
• stress in Czech is usually on the first syllable of each word:
  • HLAV-ní
  • POS-ta-va
  • MÁ
  • STRACH

Yes. Czech word order is more flexible than English word order.

The neutral order here is:

• Hlavní postava má strach.

But other orders are possible for emphasis:

• Strach má hlavní postava. = emphasizes fear
• Má hlavní postava strach? = a question, Is the main character afraid?

So the basic meaning can stay similar, but the focus or tone changes.

Yes, absolutely. That is another very common way to express a similar idea.

• má strach = has fear / is afraid
• se bojí = is afraid

They are often close in meaning, but there can be a slight difference:

• má strach often highlights the feeling of fear
• bojí se often works especially well when saying what someone is afraid of

For example:

• Hlavní postava má strach.
• Hlavní postava se bojí.

Both are natural.

And with a specific thing:

• Hlavní postava se bojí pavouků. = The main character is afraid of spiders.
• Hlavní postava má strach z pavouků. = The main character is afraid of spiders.

A very common pattern is:

• mít strach z + genitive

Examples:

• Má strach z tmy. = He/She is afraid of the dark.
• Má strach z pavouků. = He/She is afraid of spiders.
• Má strach z toho. = He/She is afraid of that.

So if you expand the original sentence, you often add z plus the genitive case.

Yes. Postava is the standard and very natural word for a character in a book, film, play, or story.

So:

• hlavní postava = main character

Be careful with charakter:

• charakter often means character/personality/nature
• sometimes it can refer to a character in a work, but postava is usually the safer and more basic choice for learners

So in this sentence, hlavní postava is exactly what you would expect.

Yes. Czech often omits the subject when it is clear from context.

So:

• Má strach. = He/She is afraid.

This works because the verb form má already tells you it is 3rd person singular:

• he has / she has / it has

Czech does this very often. If the listener already knows you are talking about the main character, you do not need to repeat hlavní postava every time.