I like Ryan's answer.
Your primary duty as a safety pilot is to act as the hooded pilot's eyes - take responsibility for flight safety with respect to those things you can see but he can't. IMO, that's not only about running into things. For example, it may also be about airspace. Is there a stadium TFR in effect somewhere near where the hooded pilot will be practicing? Are you near some other type of airspace that needs to be avoided but you folks are not speaking to ATC? Avoiding those types of incursions are part of your duties as well.
Beyond that, the best safety pilots are those who take the role the hooded pilot assigns them. But don't expect to be perfect. A friend and I have been flying together alternating SP duties for a few years. At this point, each of us knows when to butt in and when to shut up (although he's =much= better at the latter than I am). But we did start off with a rule that went, "If you see me do something really stupid, the airplane is yours."