Karl Johnson Biography: Irish actor Karl Johnson has quite an amazing list of film, television and especially theater projects to his credit. He is a regular performer for the National Theatre in London, acting in plays such as Black Snow, Uncle Vanya, The Shape of the Table, Golden Boy, The Fawn and Animal Farm. He also appeared in films such as Four Last Songs and The Tempest. Porcius Cato is played by Karl Johnson on HBO's historical drama series Rome. Porcius is the zealous leader of the conservatives in the Roman senate. Fiercely intelligent and indignantly moral, he approaches his causes with outmost passion and dedication.