Your sculpt looks great. Although it would be even better if you could change it so that it would show a typical Bill Murray expression(like the one in the top-mid). I think that would make it more recognizable.
great subject for a face sculpt study, noticed a couple things with the mouth that might help the likeness so I made a crappy paintover for ya. The show of the lips were mainly what was buggin me, and I touched up the chin and muscle bands around the mouth a little too... hope it helps!
Thanks for the crits Goraaz and Ferg. I went around and exaggerated the eyebrows, the corners of the lips, etc. subtle but very beneficial difference. I also moved the top of the chin upwards like Ferg suggested and I added some volume to the lips, although I think the size I sculpted them at was accurate to the reference…