The person "watching" should be actively engaging and helping to build the "theory of the program" (in the Naur sense) through questions and conversations. If they're just sitting there silently they aren't doing the practice. (Also, I prefer to work with both developers having a keyboard and mouse. You still should have one person driving, but the other person can jump in and directly make changes instead of having to use the other person as a puppet.)
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The person "watching" should be actively engaging and helping to build the "theory of the program" (in the Naur sense) through questions and conversations. If they're just sitting there silently they aren't doing the practice. (Also, I prefer to work with both developers having a keyboard and mouse. You still should have one person driving, but the other person can jump in and directly make changes instead of having to use the other person as a puppet.)