Possible Clues:

• Sounds like working on stage is good for muscle
• Be part of, as a movie
• Get credit for?
• Do something with current to produce muscle protein
• Be part of, as a show
• Be a cast member of (2 words)
• Be part of, with a cast
• Join the cast of, as a play
• Protein found in muscles
• Words with "self-defense"