By: +David Herron; Date: April 21, 2017
From http://www.radiotimes.com/news/2016-12-25/is-matt-lucas-in-the-next-series-of-doctor-who Nardole is being converted from a "rent-a-duffer" to someone who is providing great value to The Doctor. Moffatt is quoted saying "He’s a wee bit cheeky to the Doctor, but he’s genuinely helpful and good at things."
From http://io9.gizmodo.com/matt-lucas-will-stick-around-to-be-the-doctors-valet-in-1790136884 Matt Lucas is quoted saying “You learn a little bit more about me and I’m a bit of a thorn in [the Doctor’s] side. I have a job to do, but I think [the Doctor will] regret giving me that job.”
Matt Lucas: “As the series goes on, I think Bill and Nardole find they have more in common and challenge the Doctor more. Nardole grows to appreciate Bill and what she brings to the TARDIS. The Doctor has grown weary of Nardole but as the series goes on, I think he comes to appreciate what he has to offer. As far as Nardole is concerned, the less drama, the better. So when a human comes on board he’s not exactly delighted. He doesn’t look up to humans either. He thinks they’re of little consequence (he’s right). I think Nardole wants to stay focused on the task he’s been given and doesn’t appreciate the distraction for the Doctor that Bill provides. You couldn’t go through a whole series with him being as cartoonish as he was in ‘The Husbands of River Song’. That episode was played for laughs because it was a Christmas Special. We get to learn more about him and why he’s there. He has a purpose.”
Nardole has a "purpose" for being present, there's a "task he's been given". Nardole is not human.
Between the comedy element, he's been shown to have quite a lot of knowledge/skill. For example, the Doctor trusted him to fly the TARDIS and it was okay that Nardole had a side adventure in ancient China before arriving in Manhattan for the thing with Doctor Mysterio.
Nardole had spent time working with River Song. Did River teach him the TARDIS? What is his cultural background with which he could grok the TARDIS? Is this task from his culture? From some other organization?