QuickCheck: Does muscle turn to fat when we stop working out?

DOES this sound familiar at all - “I used to be so fit, but when I stopped working out, my muscles all turned to fat.”

Sometimes, when we see the likes of Jason Momoa or Channing Tatum, who are really fit in their respective movies, suddenly displaying less muscly builds on their social media once filming is over, it leaves a pretty big question mark on whether this is true or not.