Después del documental, habrá una charla sobre el guion y sobre las escenas más importantes.

Because del is the contraction of de + el.

In Spanish, when de is followed by the masculine singular article el, they normally contract:

• de + el = del

So:

• después del documental = after the documentary

This contraction is required in standard Spanish, except when el is part of a proper name, such as de El Escorial.

Spanish usually uses an article where English often does too, and sometimes where English would omit it.

Here, el documental refers to a specific documentary, so Spanish needs the definite article:

• después del documental = after the documentary

Saying después de documental would sound incorrect here.
If you are talking about a specific thing, Spanish normally wants the article:

• el documental
• el guion
• las escenas

Habrá is the future tense of haber used impersonally, meaning there will be.

So:

• habrá una charla = there will be a talk

This is very common in Spanish:

• hay = there is / there are
• había = there was / there were
• habrá = there will be

Even though English uses there is/are, Spanish uses forms of haber.

Because Spanish uses haber for existence: to say that something exists or will take place.

So if you want to say there will be a talk, you use haber:

• Habrá una charla.

You would not say será una charla or estará una charla for this meaning.

Compare:

• Habrá una charla. = There will be a talk.
• La charla será interesante. = The talk will be interesting.
• La charla estará en la sala principal. = The talk will be in the main hall.

So haber introduces the existence of something.

Charla is a feminine noun meaning talk, chat, or informal lecture, depending on context.

Since it is feminine singular, it takes:

• una charla

not:

• un charla

In this sentence, charla probably means a talk or discussion after the documentary, not just a casual chat between friends.

Both can refer to a talk, but charla usually sounds a bit less formal and more conversational.

Very roughly:

• charla = talk, discussion, informal presentation
• conferencia = lecture, conference talk, more formal presentation

In the context of a documentary, una charla suggests a discussion or talk afterward, possibly with audience participation.

Yes, it could be said only once:

• una charla sobre el guion y las escenas más importantes

That is also correct.

The repeated sobre is used for clarity, emphasis, or rhythm:

• sobre el guion y sobre las escenas más importantes

Repeating the preposition can make the sentence sound slightly more carefully structured, especially when the two topics are distinct.

So both are grammatical:

• sobre el guion y las escenas más importantes
• sobre el guion y sobre las escenas más importantes

Guion means script or screenplay.

In this sentence, it refers to the written script connected with the documentary or film content.

It is masculine:

• el guion

So:

• sobre el guion = about the script

In Spain, guion is the standard spelling accepted by the RAE. You may also see guión in older texts or from people who still prefer the older accentuation, but guion without the written accent is the current standard spelling.

In Spanish, descriptive adjectives usually come after the noun.

So the normal order is:

• las escenas importantes
• las escenas más importantes

not usually:

• las importantes escenas
• las más importantes escenas

Putting the adjective before the noun is possible in some cases, but it often sounds literary, emphatic, or stylistically marked. In everyday Spanish, las escenas más importantes is the natural order.

Más is used to form the comparative or superlative idea of more/most with many adjectives.

Here:

• importantes = important
• más importantes = more important / most important

In context, las escenas más importantes means the most important scenes.

Spanish usually forms this structure as:

• más + adjective

So:

• más importante
• más interesantes
• más difícil

not:

• importante más

Because it agrees with las escenas, which is feminine plural.

In Spanish, adjectives must agree with the noun in gender and number.

Here:

• escena = singular feminine
• escenas = plural feminine

The adjective importante has the same form for masculine and feminine singular, but in the plural it becomes:

• importantes

So:

• la escena importante
• las escenas importantes

And here:

• las escenas más importantes

Because the sentence refers to specific things, and Spanish uses definite articles to mark them.

• el guion = the script
• las escenas = the scenes

This sounds natural because the listener is expected to understand which script and which scenes are being discussed: the ones related to the documentary or the event.

Spanish often uses definite articles more regularly than English.

Not in this sentence.

Después de is used before a noun:

• después del documental
• después de la película
• después de la charla

Después de que is used before a clause with a verb:

• después de que termine el documental = after the documentary ends
• después de que hablen = after they speak

So here, because el documental is a noun phrase, después de is the correct choice.

It sounds fairly neutral and very natural in announcements, programmes, schedules, or event descriptions.

For example, you might see similar sentences in:

• cinema programmes
• cultural event listings
• museum schedules
• school or university announcements

So habrá una charla is a standard, useful phrase for saying that a talk or discussion will take place.