Complex sentence: Ratnam had previously wanted to cast Haasan as the protagonist in his debut film as director, "Pallavi Anupallavi" (1983), but the collaboration couldn't materialise then as Haasan was busy with "Raja Paarvai" (1981) at that time.
Split into simpler sentences: Ratnam had previously wanted to cast Haasan as the protagonist in his debut film as director, "Pallavi Anupallavi" (1983). The collaboration couldn't materialise then as Haasan was busy with "Raja Paarvai" (1981) at that time.