I worried about that sort of thing all the time. You see these epic scenes play out in your head, that get you so excited, but when you sit down to write you like "Shit... this isn't coming out right" Then you just get so frustrated with yourself that you want to throw your laptop across the room and say "Screw it" because that awesome picture in your head isnt as awesome on paper.
Don't worry about that the first draft. Just write it. Finish it, then when you re-read it you'll find yourself for many areas going "Wow, that's awesome" and for others, you might just have to add a little more detail or emotion, but with the ground work already laid, its much easier then you would imagine.
I always wanted my books to be terrifying. As i read them i realized a 6 year old wouldn't even have trouble falling asleep if they were read it. So I read up on "How to write Horror". Up the suspense in area's, added more internal thoughts, and feelings, made areas more scary. And so far's its gotten some good feedback.
The hardest part is just finishing the book. Do that first and then worry about it being perfect with your mental image, that's the easier part.
You so got this!!!!