En el estreno habrá una directora y una guionista que hablarán del guion al final de la película.

Because haber in this use is impersonal and means there is / there are / there will be.

So:

• habrá = there will be
• habrá una directora y una guionista = there will be a director and a screenwriter

Even if what follows is plural, Spanish normally keeps this form singular:

• Habrá dos invitados
• Habrá muchas personas
• Habrá una directora y una guionista

So habrán would normally be considered incorrect here in standard Spanish.

Hablarán is the simple future of hablar, third person plural.

• hablar = to speak / to talk
• hablarán = they will speak

It agrees with una directora y una guionista, which are two people, so the verb is plural.

Because the two verbs are doing different jobs.

• habrá is the impersonal existential haber = there will be
• hablarán is a normal verb with a real subject = they will speak

So:

• habrá una directora y una guionista → there will be a director and a screenwriter
• que hablarán... → who will speak...

That is why one is singular and the other is plural.

Here que is a relative pronoun. It means who, that, or who/which depending on the English translation.

It links the two nouns to extra information:

• una directora y una guionista = a director and a screenwriter
• que hablarán del guion = who will talk about the script

So the whole part means:

• a director and a screenwriter who will talk about the script

Spanish often uses que where English would use who for people.

Because Spanish normally uses the article with each separate noun in this kind of list when it means one of each person.

So:

• una directora y una guionista = a director and a screenwriter

Repeating una makes it clear there are two separate individuals, each with that role.

If you left out the second article, the sentence could sound less natural or change the nuance.

These two profession words form gender differently.

• director / directora changes ending
• guionista often stays the same for masculine and feminine

So:

• un director
• una directora
• un guionista
• una guionista

This is common with many nouns ending in -ista:

• un artista / una artista
• un periodista / una periodista
• un turista / una turista

In those cases, the article usually shows the gender.

Because de + el contracts to del in Spanish.

So:

• de el → del

That gives:

• hablar del guion = to talk about the script

Also, the verb pattern matters here: Spanish says hablar de something.

• hablar de una película = to talk about a film
• hablar del guion = to talk about the script

So del is there for two reasons:

1. hablar takes de
2. de + el becomes del

In current standard Spanish, guion without an accent is the preferred spelling.

So the standard form is:

• guion

However, you may also see:

• guión

That older spelling is still very common in real life, and many native speakers use it. But if you are learning standard modern spelling, guion is the form to remember.

En el estreno means at the premiere or at the opening showing/event.

• estreno = premiere, first showing, opening
• en el estreno = at the premiere

Spanish often uses en for events and situations where English might use at:

• en la fiesta = at the party
• en la reunión = at the meeting
• en el estreno = at the premiere

In this sentence, the most natural meaning is that they will speak at the end of the film, meaning when the film finishes / after the screening.

So:

• hablarán del guion al final de la película
  = they will talk about the script at the end of the film

Grammatically, al final de la película is modifying hablarán, telling you when they will speak.

Because al final is a very common expression meaning in the end or at the end.

Here:

• a + el = al
• al final de la película = at the end of the film

Spanish often prefers al final de... for this idea.

Compare:

• al final del libro
• al final de la clase
• al final de la película

En el final is much less common in this sense.

Yes. Literally, hablar de means to talk about, but in context it can often be translated more naturally as:

• talk about the script
• discuss the script

So depending on the context, either translation can work well.

Not only films. Estreno can refer to the first public showing or first performance of many things, such as:

• a film
• a play
• a TV series episode
• even something being worn or used for the first time in some contexts

But in this sentence, because it mentions la película, estreno clearly refers to the film’s premiere or opening event.