As long as the green bar that floats around above my head isn't empty, I am at peak physical condition and will never ever tire or sustain visible injury, even when shot, stabbed, or eaten alive.
The moment that bar is empty, I will do one of three things:
1. Fall down/crumple over/spontaneously explode, phase in and out of reality for a few seconds (if I did not explode), then be replaced by someone who looks and acts exactly like me. Note that whether my replica appears in the exact same position of my untimely demise or at the beginning of wherever I was attempting to invade before shrugging off my mortal coil may vary.
2. Fall over and die, usually with some dramatic line or death cry. If I was hit hard enough, I may even fly across whatever room I was in before slamming into a wall, though this idea is just as often ignored. Note that if I am required by some strange metaphysical force to fight again in a sort of "Round 2"-type situation, I will immediately stand back up and the bar above my head will completely refill, which of course restores me to peak physical condition.
3. If I am designated as an "enemy", or only refer to myself by a general type or species name, I will either act according to reaction 1, except without any chance of instantaneous replacement (sometimes averted), or simply disappear without any sign of actually suffering a horribly painful death.