Their music-themed movie is to open Friday.
Houston accidentally drowned in a hotel bathtub last February after using cocaine. She was 48.
"I couldn't handle it," Sparks told "Nightline" in an interview to air Monday night, discussing how she got a call telling her Houston had died. "I was like, 'You're lying, you're lying to me.' So, we were like: 'OK, well, let's turn on the news. If it's there, then maybe that's confirmed it.'"
Sparks said she and Houston had been scheduled to appear together on the red carpet at the Grammy Awards the weekend she died.
"There's just no reaction," Sparks said. "I couldn't move. I couldn't breathe. I couldn't say anything. I was in complete shock, and somebody had to sit me down before I fell over. And I just lost it. I lost it completely."
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