If I were a character in an action movie, I'd be that peripheral character - the one that has a quirky personality and has access to all of the best technology. The guy that the main character depends on when things get tough, who never gets the girl even though he wants her more than the main character, and who dies halfway through - giving the main character enough desire for revenge that he succeeds against all odds and finally defeats the enemy. I'd probably be dying of cancer, except the audience can't feel sorry for me because I smoke like a chimney. Or maybe I work with radioactive material. Or maybe I'm 900 years old I am.
Either way, it's my fault I'm dying so you don't have to feel sorry for me.
The audience loves the main character more, because he's vulnerable. Because he has faults. He's overcoming those odds, which makes him heroic. I was given those strengths before the movie started, which makes them normal to me, even though the main character can't do what he accomplishes without my extraordinary technology / serum / hacking abilities / fuel for revenge. I don't have faults, which means I can't be heroic, no matter how many times I show up and save the main character before I finally get killed by the bad guy in an inexplained moment of weakness.
In other words, I may show the main character what he has to do to become heroic, but I am "that guy", so you don't have to admire me. That's just my role in the whole saga.
If I'm lucky, the movie will end with me being reunited with the main character, like in Die Hard, where I was the plucky limo driver. I might even go on to get my own sitcom afterwards.
Probably though, I will have to die to motivate the plot like the character "Whistler" in Blade, or Obi Wan Kenobi or Yoda on Star Wars.
