Of course it's heavy, that's why they call it weight. Feel Free to improvise on the above list. Screaming at your partner with a fevered intensity is important. Again, see Full Metal Jacket for details. The louder you scream, the better.
Public humiliation can be extremely motivational.
If your partner can talk during a set, or right after it, they were not lifting with psycho levels of effort and concentration. Up the weight, force more reps, and use motivational phrases to increase the mental and physical focus of the workout. Know your partner, and know his limits, push him beyond those limits, and insist that he do the same. Again, use the above effectiveness gauge to judge if they proper effort is being put forth. Demand total effort, be uncompromising.