All modifiers affect the attribute, not the dice roll. They are summarized in the following table. They should, however, be reserved to the most extreme situations imaginable. Famous actors often have roles in which they can accomplish unbelievable feats. The rules reflect this by giving PCs bonuses to their attributes according to their status.
Extras get no additional abilities.
For most other things he would use normal, unchanged levels. If modifiers lower the chance of success below 3, a roll of 2 will still succeed, but it will not be a critical success. Likewise, if the chance is over 11, a roll of 12 will still fail, but it will not be a fumble. Often, when a PC attempts something, someone will want to resist it. Combat is the most common example.