Baa! Who wants to be in a Christmas play if they make you be a sheep? A sheep doesn't get to do anything.
9 seconds sound clip from the It's Christmastime Again, Charlie Brown movie soundboard.
You can hear this line at 00:18:58.158 in the Blu-ray version of the movie.
Quote context
[...]
- You're a sheep, sir. All you have to say is, 'Baa.'
- My mind has gone blank. I'm doomed. I wonder if this happened to Laurence Olivier?
- Looks like the play will start soon.
- Well, you'll have to excuse me, sir. My first scene.
- Hark.
- I am Gabriel. Do not be afraid, Mary.
- Behold, I am the handmaiden of the Lord.
- I am Gabriel, Mary, and I couldn't hear you because of the sheep.
[...]