Learn Arabic (Modern Standard): صرخ الممثل البديل صرخة عالية خوفا من السقطة. - The stuntman shouted a loud scream out of fear of the fall.

Both صَرْخَة (a scream) and السَّقْطَة (the fall) are Nouns of Once. They take the roots for shout (ص-ر-خ) and fall (س-ق-ط) and put them into the فَعْلَة pattern to show the action happened exactly one time.

What are the regular verbal nouns for these actions, and why aren't they used here?

The general verbal noun for shouting is صُراخ and for falling is سُقوط. The Nouns of Once (صَرْخَة and سَقْطَة) are chosen instead because the sentence describes one specific scream regarding one specific fall, rather than the general concepts of screaming or falling.

If صرخ already means he shouted, why do we need to add صرخة right after it?

This is a structure called the absolute object. By adding the noun version of the verb (صرخة), it gives us a grammatical place to attach the adjective عالية (loud) to describe how he screamed. Literally, it translates as 'he shouted a loud scream'.

Why does خوفا end with an alif and an 'an' sound (tanwin fatha)?

It acts as the object of reason. In Arabic, you can use a noun in the accusative case (which takes the tanwin fatha and an extra alif here) to explain why an action was done. So خوفا means 'out of fear' or 'because of fear'.