How do I keep a murder-mystery dinner moving when a guest won't reveal a scheduled clue?

The second envelope told my cousin to read her clue aloud. She tucked it under her plate and said, 'My character would never admit that.'

I'd bought a murder-mystery dinner pack and invited eight people. The instructions said clues in those envelopes should be revealed at that stage. I'd explained that before we started. She was enjoying playing a secretive heiress, while the other guests were waiting for information they needed to make their guesses.

I asked her to read it in character. She said following a script so closely defeated the point of acting. I ended up reading the clue myself, and she barely joined the rest of the game.

How do I apologise for taking over her moment without accepting that one person's performance can stop everyone else's mystery? I'd like to host again, but with a clearer agreement about where improvising fits.