Charlie Murphy had his down days on the set of "Black Jesus" but NEVER complained about his health issues ... according to his co-star and friend Gerald "Slink" Johnson.
Slink, who plays Jesus, says he's heartbroken Charlie mostly kept his leukemia battle to himself. As we reported ... he did the same while shooting "Power" with 50 Cent.
But Slink said the thing he'll remember most about Charlie is his relationship with his 3 children.
TMZ broke the story ... Charlie died Wednesday in NYC at the age of 57.