People often obsess about how much weight they are using. They know what too much weight feels like - they can't lift it! - but aren't sure how to find the right weight.
Put simply, the weight you lift is just a tool, but you need to use it to your advantage. You need to use enough weight that you can only just complete a set in good form. If you have to squirm around and compromise your form to finish a set, you're using too much weight. If you can crank out 12 reps easily, it's time to up the weight, decrease the rest time between sets, or increase the number of sets and or reps you perform.
I'll repeat that - the perfect weight is one where you can only just complete the last rep in the set - one more would be impossible. Indeed, if you really want to grow, you need to train to failure on a regular basis - that is, perform sets where you can't actually complete the last rep in good form.
So if you get the weight right, everything else will fall into place - which brings us back around to intensity.