How important is physical resemblance?

Not so much.
It's all about the performance and how the actor or actress disapeears into the Person they are portraying with a good script and sometimes research of the character.


The example I can talk about is Geoffrey Rush as the Marquis DeSade.
He didn't look a bit as the marquis we believed he was the marquis thanks to his acting and the script he was working with.

Not very important.

James Cagney didn't remotely look like Lon Chaney, Sr., but gave an excellent performance.

Sometimes an actor need only adopt a particular characteristic of their subject (a mustache, glasses, etc.) to give a surface impression, but the acting is paramount and no level of physical resemblance can equal that.

To God There Is No Zero. I Still Exist.

I think the performance is more important than the resemblance.

Volker Flenske: (While torturing David) I don't know why you're doing this to yourself!

With the kinds of makeup available today, and the countless hours of films and interviews available on many biographical subjects, it's possible for today's actors to give highly accurate impersonations.

In my case, self-absorption is completely justified.

I take value in resemblance. I mean, I don't wanna see just any ole body playing in a biopic. I wanna enjoy the person portrayed on all levels.

Check this indian film Dangal. What a terrific transformation this 51 year old actor did for a biopic.