Can you use Not So Fast to target Overzealous Fan's ability?

Ruling: Yes, you can use Not So Fast to target Overzealous Fan's ability because it targets a friendly unit. However, if you use Not So Fast, the opponent will never have the choice to kill Overzealous Fan.

Sequence:

• Not So Fast can only be used before Fan's trigger resolves
• If Not So Fast is used, the ability is countered and the opponent never gets the choice to kill Fan
• If the ability is allowed to resolve, the opponent can choose to kill Fan to move a unit, but there is no opportunity to use Not So Fast at that point

Nuances:

• You cannot wait to see if the opponent chooses to kill Fan before using Not So Fast - the window to use Not So Fast is before the trigger resolves