Agree.
Great actors and actresses become the character and make the audience believe they are the character.
Too may of the modern ‘actors’ and ‘actresses’ are, as you say, portraying themselves, with roles written for them. They are actually personalities, not ‘actors’ and ‘actresses.’
“Personalities”. You nailed it, that’s the word I was looking for. From the Lord down to Daisy, the kitchen maid, each character is utterly believable. Each character is essential and fascinating. I especially liked the way Matthew bailed out Tom and vice-versa, an incredible bonding between “classes”.