(fallen london stamps by an odd habit of the good doctor's: he tends to touch his close friends a lot, unless they don't want to be touched. so, if you're close to him, and amenable to it, he might do stuff like lightly grip your shoulders with the tips of his fingers, or cheerfully tap you under your chin if you're taller than him, or pat your head if you're shorter
or, if the two of you are deep in a conversation, he might reach over and gently take one of your hands, and hold it enclosed in both of his - one below, and one above.
it's almost as if he's reminding himself that you're real. that you're still there.