Throughout his presidency, Mr Chavez has had a close friendship with Mr Castro, describing him as being like a father and mentor.
Mr Castro has seen Mr Chavez every day since Tuesday's operation in Cuba, science and technology minister Jorge Arreaza said.
"He always stops by to personally find out about El Comandante's health condition and also to share his knowledge with all of us, and to give the family courage and encouragement," the minister said.
Mr Arreaza, who is Mr Chavez's son-in-law, said the president has been with his children since the operation, which was his fourth cancer-related surgery since June last year. Mr Chavez has been making "favourable" improvements, Mr Arreaza said, calling it a "process of stabilisation".
The Venezuelan government has said Mr Chavez suffered complications during a complicated six-hour operation but he has since been recovering progressively.
Mr Arreaza said there had been "moments of tension, above all on December 11 and 12, but we've overcome them one by one".
"El Comandante has fought a great battle, and as he told us he will overcome each of these moments little by little. He's getting better," said Mr Arreaza, the husband of Mr Chavez's eldest daughter Rosa. Mr Arreaza said that as Mr Chavez began to talk after the operation, the president told him of "his deep love for his people".
The 58-year-old Venezuelan leader underwent his fourth cancer-related operation after tests found the illness had come back despite previous operations, chemotherapy and radiation treatments. Throughout his treatments in Havana, he has kept secret some details about his pelvic cancer, including the exact location and type of tumours removed.
Venezuela's government has said Mr Chavez suffered complications during a complicated six-hour operation but has since been recovering progressively.