I have thought of a few options, but none of them convinces me 100% because obviously nobody with Brienne’s physique would get hired in old Hollywood, unfortunately, but at the same time it’s such an important aspect of her character that I can’t really leave it out. Usually I like to keep things secret until I do the fancast post, but in this case I guess I’ll share my options so you guys can give me your opinion and maybe help me decide.
Option 1: Jean Arthur

I’m biased towards this one because Jean is one of my favourite people ever, and she could get across that feeling of awkwardness that Brienne tends to have. Of course, she’s way too pretty, but that’s the case for all of the potential Briennes.
Option 2: Miriam Hopkins

The problem here is that Miriam is very short and petite, and if you’ve seen any of her films you really get that impression.
Option 3: Margaret Sullavan

Ok, I’ll be honest, this is mostly because of that one pool photo.
And then there’s this:

This would solve the height problem since Ingrid was rather tall, but at the same time she’s probably the most conventionally attractive of the bunch. Also I think I might use her for some other character too.
So, what do you think? Would any of these work?