The 'Fear Factor' can be one of the biggest rushes in a scene BDSM in general. The slightly sick, out of body feeling when you hear that tone of voice and the build-up has been just right. The looming realisation that something's going to happen and it's going to hurt and you deserve it. For me the fear factor is greatest in role play scenes as in reality tangible fears don't hold much sway over me (but don't get me started over the intangible fears). Be it the strict headmaster, the reformatory warden who is clearly a real sadist or a vengeful slave master someone people who can go with me to a place where I'm genuinely scared and subsequently repentant or at the very least cowed are wonderful.
There was a time in the not so distant past when the fear factor rose more quickly. In a couple of very memorable scenes when I was first dipping my toes into the waters of 'the scene' and BDSM as opposed to kinky games with a boyfriend I actually fainted. The fear (and shock) factors of being in a real dungeon with real implements I'd only heard about in books with someone I barely knew scared the hell out of me but in a really hot way. In all honesty the actual fainting wasn't hot but that level of nerves whenever someone so much as spanked me was.
The fear factor doesn't necessarily kick in because you're going to play really hard - it's a mindset. It's the build up, the uncompromising tone of voice, the unyielding nature of having to be in a certain position or respond in a certain way, the relentless hand forcing your head further down and the associated feeling of helplessness. The indicators that someone means business kicking in and you senses hitting a heightened level meaning less can even be more. I love it and the sick feeling in my stomach that comes with it.
Thinking about recent play it's not something that has been kicking in so much. The boy interest and I are experimenting with Ds and playing more as us (which is both scary and wonderful in a completely different way) and I don't seem to have got round to playing that type of imbalanced mind-fuck of a scene. This led to me thinking about why and I think part of it is being rather stressed with real-life issues and physically away, which has left me with less time to plot evil scenes and ferment bad behaviour - I haven't been playing so many 'formal' scenes and those I have played have been hot and fun rather than scary. I also have been too taken with the boy to meet new people. I'm not sure if you tend to have these scenes more with someone new but I think sometimes it can make it more scary but then as Emma Jane rightly pointed out if a scene is going to push your limits as part of the fear factor the trust of being with established playmates is absolutely crucial. Thinking about it has given me the urge to plan some scene that will push the 'fear factor' as looking back those scenes have historically been some of my favourites (and I may write about a few) but I throw the floor open - what gives you the 'fear factor'?