Başrol oyuncusu sahneye çıktığı sürece sahne korkusu azalacak.

Başrol oyuncusu means the lead actor / the main-role actor.

It breaks down like this:

• başrol = leading role
• oyuncu = actor / performer / player

So the whole expression is a very natural way to say lead actor in Turkish.

Because çıkmak here means to go up onto / go out onto, and Turkish marks the destination with the dative ending.

• sahne = stage
• sahneye = to the stage / onto the stage

So sahneye çıkmak is the standard expression for to go on stage.

Compare:

• sahnede = on the stage or at the stage
• sahneye = onto the stage

Since the action involves movement toward the stage, -e / -a is used.

Çıktığı is a subordinate-clause form made from çıkmak.

Very roughly, it comes from:

• çık- = verb stem
• -DIK = a suffix used to form a clause based on the verb
• -I = a possessive/person marker here showing third person singular

So çıktığı means something like that he/she goes out/goes up in structure, but in this sentence you should understand it as part of the fixed pattern çıktığı sürece.

A few sound changes happen:

• çık + dık becomes çıktık because d becomes t after a voiceless consonant
• then with the vowel-initial ending, the final k softens to ğ
• so you get çıktığı

You do not need to translate çıktığı by itself word-for-word here; it works together with sürece.

The pattern verb + -DIğI sürece means:

• as long as ...
• so long as ...
• for as long as ...
• sometimes provided that ..., depending on context

So sahneye çıktığı sürece means as long as he/she goes on stage or for as long as he/she is going on stage, depending on how the sentence is being interpreted in context.

This is a very useful Turkish structure for duration or condition.

Examples:

• Sen çalıştığın sürece başarılı olursun.
  As long as you study/work, you succeed.

• Yağmur yağdığı sürece dışarı çıkmayacağız.
  As long as it keeps raining, we won’t go outside.

That is a very common question.

In many Turkish subordinate clauses with -DIK, the subject can appear in the genitive. But in adverbial patterns such as -dığı sürece, Turkish often uses the subject in its plain form as well, especially in natural everyday usage.

So başrol oyuncusu sahneye çıktığı sürece is normal.

The person/number is already reflected on çıktığı, which is why the sentence still works clearly.

A learner should mostly remember this as a set adverbial construction:

• ben geldiğim sürece
• sen çalıştığın sürece
• o beklediği sürece

No. Here -su is not showing possession in the English sense of his/her.

Sahne korkusu is an indefinite noun compound, a very common Turkish structure:

• sahne = stage
• korku = fear
• korkusu = the compound form used after another noun

So sahne korkusu means stage fear / stage fright.

This pattern is extremely common:

• araba kapısı = car door
• ev sahibi = home owner / host
• çocuk kitabı = children’s book

So the ending on the second noun is just part of the compound structure.

The subject of azalacak is sahne korkusu.

So the sentence structure is basically:

• Başrol oyuncusu sahneye çıktığı sürece = adverbial clause, giving the condition/time frame
• sahne korkusu = subject of the main clause
• azalacak = main verb, will decrease

That is why the sentence is not saying that the lead actor will decrease. It is saying that stage fright will decrease.

Azalacak comes from azalmak, which means to decrease, to lessen, or to go down.

• azal- = decrease
• -acak / -ecek = future tense

So azalacak means will decrease / will lessen.

A useful contrast:

• azalmak = to decrease (intransitive)
  Something decreases by itself.
• azaltmak = to reduce (transitive)
  Someone reduces something.

So here:

• sahne korkusu azalacak = stage fright will decrease

not

• someone will reduce the stage fright

Because the two instances are doing different jobs:

• sahneye çıkmak = to go on stage
• sahne korkusu = stage fright

Even though English might sometimes try to avoid repetition stylistically, Turkish often keeps repeated words if they belong to normal expressions. There is nothing odd about that here.

Yes, Turkish word order is flexible, although not random.

The given sentence is natural because it starts with the condition/time expression:

• Başrol oyuncusu sahneye çıktığı sürece sahne korkusu azalacak.

You could also say:

• Sahne korkusu, başrol oyuncusu sahneye çıktığı sürece azalacak.

That version puts more focus on sahne korkusu.

In Turkish, the main verb usually stays near the end, and earlier parts of the sentence can move around for emphasis or information flow.

They are similar, but not the same.

• çıkınca = when / once he goes on stage
• çıkarken = while he is going on stage
• çıktığı sürece = as long as he goes/is on stage, with an idea of continuing duration or condition

So çıktığı sürece is the best choice when the meaning is about something being true during the whole period or under that continuing condition.

It is less like a single moment and more like an ongoing frame.