[Andrew Hackard]

This may not be explicitly specified in the rules, but it's such a fundamental part of game play that I am making this official ruling here and now: You cannot play cards that are not under your control.

This means, unless a rule explicitly says otherwise, you can't play cards out of the deck or discard pile and you definitely cannot force an opponent to play their cards. (And by "explicitly," I mean it has to say exactly that, not maybe perhaps somehow allow it by some tortured reading that is clearly outside the spirit of the game.)