Yeah, the trick with both liquid smoke and fish sauce is to use way less than you think you need. You'll notice I'm only putting a teaspoon in about a full cup of marinade, and that's enough to give a very distinctly smoky flavour. You can easily cut that back to a half or quarter teaspoon if you prefer it more subtle.

For fish sauce, I'm going to start out with the same kind of measures. I'll probably go with 1tsp and then adjust up or down from there. If you can taste the fish sauce in the finished product you've used too much. And yes, brand absolutely matters. I only ever use Squid brand fish sauce personally.