V Bell has a article in the Observer (here) on the scientific study of hypnosis.
The easy of suggestibility is partially inherited and is spread across the population in a bell curve (like height). You are easy to hypnotize or hard or in between. You can go into a deep trace or be only mildly affected.
There are differences in the brain that correlate with suggestibility. Scans can tell good faking from real hypnotic traces.
“it was possible to “switch off” automatic word reading and abolish the Stroop effect – a psychological phenomenon that demonstrates a conflict between meanings, such as where we are much slower to identify the ink colour of a word when the word itself describes a different hue. Furthermore, when this experiment was run in a brain scanner, participants showed much lower activation in both the anterior cingulate cortex, an area known to be particularly involved in resolving conflict between competing demands, and the visual cortex, which is crucial for recognising words. Although this may seem like a technicality, to the scientific world it was a strikingly persuasive demonstration that hypnosis could apparently disassemble an automatic and well-established psychological effect in a manner consistent with the brain processes that support it.”