For me the deciding factor is that because the perform a stunt card has only the basic trait, not the active trait, it simple cannot be used to do anything active, period. A player is never permitted more than one standard per turn. All other actions would have to be valid out of turn plays, such as a card with an active trait.
As far as uses of the perform a stunt card during a turn, it is meant to be very open ended allowing actions that they would otherwise qualify. It would not let someone of Str 2 toss a heavy door aside with one hand, while carrying the princess under his other arm, but if his Agility was 5 he could surely use perform a stunt to crash through the window next to the door, landing in a way that he shields the princess from harm, taking a few wounds himself as he slides across the broken glass.
It may sound like it lacks a little imagination, but I think it makes perfect sense to allow it to be used to emulate other action cards, with some caveats. For example, maybe you have seen your soldier buddy do a nasty double strike when pressed by goblins, and you find yourself surrounded and think to try it yourself. Why not?
- You must otherwise meet the qualifications required to use the action
- You have no training..there are more ways to do it wrong, and more ways to be unlucky about it.
- Add 1 challenge and 1 bane to the pool.
- Can only convert comet's into boons. (Cannot activate specific card comet effects or convert to success, due to lack of training or practice.)
- If this sounds too open ended, restrict it to major story moments..like a depserate move to save a friends life, or get across a chasm before its too late, etc.
Ultimately, if a player really likes an action, he'll probably get it at some point, and this allows you to say "yes" to your players in a way that lets them experiment or act out of story-driven desperation.