As Haz said, it's completely cliched and character development is crucial. And no, character won't come through dialogue. Your dialogue needs to reflect each character, therefore they need to be clear in your mind before starting. Especially for characters you described above.
As for your "this is mainly an action movie, with a slim plot", you may want to rethink that. It's okay for Jerry Bruckheimer to make an action movie with a slim plot because he's Jerry Bruckheimer. But no one in their right mind would buy a script from an unknown unless it has an awesome plot. Plot drives the script in stories like this.
Despite all I said, you should work it out. There's nothing cooler than finishing a screenplay you've come up with.