It probably isn't random, but I've read so many different opinions that I can't be sure. I'll try the various outcomes on my hard mode run.

I forgot about Prayer, but I wouldn't recommend it since it costs too much to use… In fact, I don't recall ever having two ATB bars to use, unless the enemy was already staggered and my active character (either Cloud or Tifa most of the times) was beating him senseless. So, a non-active character with two full ATB bars is something I can only dream of (perhaps with the right combinations of Complete materia)!