Question about my on board flash on Rebel XT

I'm rather embarrassed to be asking this question, but here goes.

I usually shoot in aperture priority and if I need some fill flash, I adjust my FEC and manually pop up my flash.

My question is: If I have my lens wide open and I have enough available light and I manually pop up my flash but don't adjust my FEC (leaving it at 0), will the flash still fire even if there's enough light?

Does that make sense?

I also want to know if when I'm shooting in AV mode and I manually pop up my flash without adjusting my FEC, will my camera fire the correct amount of fill flash needed? In other words, would the camera's assesment of amount of fill flash needed be the same as mine?

I hope I'm making sense

Yes for both questions.

If you ‘force’ flash to open in Av mode, camera will still fire a flash even if there is enough light, and yes, ETT-L will try to expose the subject properly.
However, the background will be lit by available light.