Bill Cosby speaks to ABC News about late co-star Malcolm-Jamal Warner

Cosby said he and Warner stayed close after "The Cosby Show" days.

Bill Cosby shared heartfelt memories of his on-screen son, Malcolm-Jamal Warner, who died Sunday after drowning in Costa Rica at age 54.

Warner, best known for his role as Theo Huxtable on "The Cosby Show," was caught in a strong current while swimming during a family vacation on Costa Rica's Caribbean coast. Despite rescue attempts by beachgoers, first responders were unable to save him.

In an exclusive interview with ABC News, Cosby remembered Warner, who played his son for eight seasons on their hit show, as an ultimate professional.

"He was always a great studier, and I enjoyed working with him very much," Cosby said.

The role of Theo was loosely based on Cosby's own son, Ennis, who passed away in 1997. Cosby revealed that he and Warner maintained a close relationship long after the show ended.

"Malcolm calls here regularly," Cosby said, describing their relationship as one where he acted as a mentor, similar to his own grandfather.

"He was very professional," Cosby recalled. "He always knew his part... He always knew his lines, and he always knew where to go."

Warner's career flourished beyond his role as Theo Huxtable. He appeared in numerous TV shows, including "Sons of Anarchy" and "The Resident" and even won a Grammy for Best Traditional R&B Performance in 2015.

His most recent work included a podcast exploring Black experiences and identities in America.

Tributes poured in from entertainment industry colleagues, including actress Tracee Ellis Ross, who said Warner "made the world a brighter place" and Jamie Foxx, who wrote he was "speechless," adding "Rest in power, my brother."

Bill Cosby has been accused by multiple women of sexual assault and sexual harassment. He served three years in prison after he was convicted of sexual assault in 2018 -- but that conviction was overturned in appeal. He has maintained his innocence.