Hi, I'm trying to make sims from the movie Pacific Rim since I haven't seen anyone else make any and I'd like some advice on my face-sculpt. This is Dr. Newton Geiszler as played by Charlie Day.

I don't know who this actor is, but I think you did a pretty good in his representation to sim. I did, however, notice a things, minor things, but since you asked, and it could be just the angle the actor is lookin (slight upwards angle) compared to the sim.

(1) Actor upper lip looks a bit higher than the sim
(2) Actor seems to have , umm how do I say this, a bit more upturned nose/ nostrils showing than the sim does.
(3) Actor seems to have, umm, the creases around mouth and cheeks, think they're called laugh lines, and his cheeks seem a bit .. umm puffier, but only slightly.

Is the actor's skin colour possibly a fraction darker than the Sim's? Perhaps S2 rather than S1? (It might just be the lighting.)

Seems like a good start but you're not quite there yet.
My comments:
The face doesn't look round enough at the jawline. The eyebrows aren't quite right either, they need a bit of an arch in them, but it's hard to tell with the glasses in the way. The nostrils need to flare out more at the end and the tip of the nose needs to be more bulbous. The ears need to stick out more too.

Thanks for the advice! I've worked on him some more (better outfit, among other things) — any more ideas?

You can easy your work with a picture,where the actor is facing the camera(and your sim). Just paste half of their heads and observe the differences. This is how i do it when i make celebrity sims. Also, the skintone idea wasn't bad,see it in game and compare