Love this topic! After thinking for a while and reading all the other comments (while simultaneously watching ET) I realized my answer for my favorite bit part actor was staring me in the face: Dee Wallace (Mom on ET; Reid's hypnotist in Memoriam).

That part could have been so overdone or cheesy or unbelievable, but Dee really brought a level of sensitivity and humanity to the part. I particularly like the way she talks Reid through the actual hypnosis.