I agree that Thewlis is perfect for Lupin. The thing that bothered me the most was that mustache *shivers*. Now, I'm not a mustache hater, far from it. My father had one most of my life and my dh has had one as long as I've known him. But I just don't like it on Thewlis/Lupin.   And yes, while he's not conventionally handsome he's still got it as far as I'm concerned.  
I loved him in Dinotopia, he played the slimy, greedy Crabb well, and he absolutely gave me the creeps in Dragonheart, but I just adore him as Lupin. I think he really nailed the character. I can't wait to see him in New World. And I must see Restoration at some point as well because he looks sooo dashing in the screen caps! 
Did you know he writes poetry as well? 
Fly 
Where, and when, and if, 
I die 
I desire to revisit this filth 
As a fly 
and on some squalid afternoon 
Fly smack into your bathroom, 
Small and black, 
And crawl all over 
Your naked young back. 
I know you imagine I'm a sensitive man, 
But I'm afraid that's just 
The kind of fly 
I am. 
~David Thewlis