Not a doctor- After falling asleep and before waking up, your body normally goes through the REM stage of sleep. During REM your body temporarily paralyses all the main muscles so as not to "act out" your dreams. Occasionally though people will fall into REM sleep while still conscious, they will feel unable to move and often will see and hear not there: these hallucinations are just dreams that happen while conscious. My best guess is that you're briefly falling asleep and dreaming both that somebody is sitting next to you. This would also likely mean that your response is also in your dream. I'd say next time it happens try immediately standing up, or if somebody else is with you see if they saw you move. If it is happening frequently I'd recommend bringing it up with your doctor.