Like a blank piece of paper or a blank document, a blank canvas is frightening, only bigger.
I generally paint large: on a 30 x 40" or 24 x 24" canvas, with big hardware store brushes. So how will I possibly fill up the space? By the time I get started, I have the opposite problem, and have to edit the composition to get it all in. So what gets me going on a new painting? Like a musician playing a few familiar chords before starting to riff, I need to start with a process. In other words, I need to build the springboard before I can jump.
Starting with Random Thoughts: Most artists "tone" a canvas before they start painting -- covering the entire surface with a middle value tone to use as a baseline for what comes next. I once ran out of paint when on location and toned a canvas with the remnants of my palette. I had 5 wet colors left on the palette and the result of my "toning" was splotchy and random. But it had a life of its own, and started the direction of the painting for me.
Sketching out the planes, skipping the details: After toning a canvas, I'll take a big stick of charcoal and draw large, messy planes of perspective -- they usually look like huge "X"s and determine where the focus and tension will be. I never outline the shapes of what I'm going to paint. Once I draw too much, it will skew my vision and I'll never paint the big picture.
Destroying for the Sake of the Greater Good: If a part of my painting is great, but doesn't work with the overall concept, I will paint over it. It hurts, but paintings don't work unless the whole is much more than the sum of its parts.
Turning Everything Upside Down: I turn my paintings constantly as I paint -- that's the only way not to get stuck in the details and know if the composition is working as a whole. Let it Rest: Although most of my works have 40-50 layers of paint, I'm not going to get an A for effort. If my mind is blocked and I can't think anymore (painting is decision-making on steroids: a decision every 30-45 seconds, everytime I mix a color or lay down a stroke) I have to turn the easel around and walk away. I'll think differently when my mind is clearer.