Well, Valium isn't really used to treat depression. Rather, it is used to treat anxiety. If a non-anxious person were to take the drug, it would probably mellow them out a bit.
Prozac, on the other hand is an anti-depressant of the SSRI type. If a non-depressed person were to take this medication, they'd probably feel a little bit of additional anxiety added because the intent of this medicine is to "boost" a person out of their depression while making adjustments to the brain chemicals. However, you have to remember that Prozac takes about 8 weeks before it "kicks in" and starts taking effect. Valium, on the other hand, will display results within an hour of consuming the medication.