For a regular sound triliteral, فَاعِلٌ is more than a change of final case ending. An alif is inserted after the first radical and the second radical receives kasrah.
Word / formMeaning / explanation
كَتَبَ ← كَاتِبٌwriter.
تَرَكَ ← تَارِكٌone who leaves.
مَنَعَ ← مَانِعٌone who prevents.
Recognising the radicals gives the pattern; number and gender endings are then added to the participle.