does fade behavior use opacity or vislibility or b

When you use the Fade behavior "start fade" the object becomes invisible.

I am having trouble making it visible again. I tried setting it to visible, didnt work, tried setting its opacity to 100, get strange effects.

Does fade work on opacity only, or does it set the "visible" flag, or some other mechanism?

It only changes the opacity - it's not possible to make an object appear part-way visible using 'set visible' or 'set invisible'. What strange effects did you get?