Craig Charles does this a lot. In series six (can't remember the episode) he mouth Cat's line 'We're deader than A-line flares with pockets in the knees.' You can also see him smiling when he's supposed to be unconscious in Confidence and Paranoia.
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Rissa writes:
And in "Backwards", he mouths the end of the Cat's line "Before you know it, you're singing soprano in the school choir."