If you manage to watch any interview with David Threlfall you reliase exactly how good he is at acting. In his Shameless role he portrays an intelligent, but lazy man with an opinionated outlook on the world. He's an awful, selfish father who's talks without thinking and with very little regards for others.
In reality, he appears to be nothing like this.
For any man to make such a transformation is a huge feat. I know that's his job, but he does it fantastically well. He's believable in all of his acting roles (or at least the ones I've seen).
For me, David Threlfall makes Shameless what it is.