It's a good sign that, even while portraying ultimate evil, Tobias can wear a uniform with dignity. This will come in handy.
He can also take a moment to DISAPPROVE OF THINGS while wearing said uniform.
And he can stand next to a very accomplished lady and act as though he's relatively proud of her, even if he is also counting down the seconds to crawling into a brandy and bantering with someone else. (That's in the Outlander-verse, of course, but I believe Phil IS a flirt._
He can look tipsy, and very sad, both at his own folly and that of the people who keep letting him down.
He can cry into his wife's face, which... not a skill he'll need on The Crown, but it's nice to know he has it.
He can look a bit put-off by whatever shenanigans are forthcoming.
He can look mischievously amused.
He can seem like he rather doesn't understand why we're all still mucking about here if you've already gotten your photos.
And he doesn't seem like he ENTIRELY loves standing on front of a photo of himself; Philip seems like he'd have no time for that either.
And look! He is already tight with King George VI himself. Can't get a better job reference than that.
Olivia Colman and Hugh Laurie
This was the original casting rumor, by the way: Hugh Laurie for Philip. He was in The Night Manager with Olivia Colman. As was, indeed, Tobias Menzies, so maybe we can just get that entire cast over here. Jess already suggested Elizabeth Debicki for Diana, and surely The Hiddles can pop by as... oh, I don't know, that gadabout Andrew, maybe?
I don't have a pic of these two together so I had to make one. Presumably he's going to have to dye his hair...? It'll be intriguing.