Rafe Spall explains why he went back to school to play a troubled maths teacher in his latest film - and Jeanne Craig discover why it all added up

RAFE SPALL'S dishevelled, scruffy-bearded character in new film X+Y wasn't intended to be a figure of desire, but try telling that to his other half.

The Black Mirror star's actress wife Elize Du Toit found his depiction of troubled but good-hearted maths teacher Mr Humphreys "much more attractive" than his toned and clean-cut alter ego in the 2013 romcom I Give It A Year.

"I've done parts before where everybody involved is willing you to be as handsome as you possibly can, but that's not the case in [X+Y], which was kind of a relief," the actor says with a laugh.

"[Humphreys] is lovely, messed up and pained, but kind. I think that's what people respond to, his kindness."

Spall, 31, was also happy growing a beard for the role. "I was actually quite trendy in my scruffiness. You need a beard now, if you're going to drink expensive coffee and wear utility clothes, so I enjoyed it..."

The moving, and at times heart-rending, feature film debut of documentary-maker Morgan Matthews, follows Nathan (Asa Butterfield), a teenage maths prodigy who is on the autistic spectrum and struggles to form bonds with others, including his widowed mother Julie (Oscar-nominated actress Sally Hawkins).

He's taken under the wing of Mr Humphreys, who recognises Nathan's gift for numbers and helps him land a place on the UK team at the International Mathematics Olympiad (IMO).

We see Nathan build relationships and get confronted by unexpected challenges, as he travels with the team to a training camp in Taiwan, and then returns to England for the contest.

Mr Humphreys has his own struggles too, having been diagnosed with multiple sclerosis [MS].

"Where we find [Humphreys] in the film, his life has not gone to plan," says Spall, who researched MS in detail and met people with the condition.

"He had huge promise as a mathematician, but when we find him, he is a lonely bloke who has now got multiple sclerosis, which is a mysterious disease, and he has not achieved all that he should have done in his maths career."

As his condition gets worse and worse, the teacher "self-medicates" with alcohol and marijuana, but his work with Nathan and his growing closeness with Julie provide some respite.

"Julie is extremely grateful to Mr Humphreys because she is trapped in a world where she looks after her son, and that is all-consuming," the actor notes. "People are always defined by how they deal with adverse situations and when you get two people dealing with difficult things, it can make for a lovely relationship."

Spall - the son of actor Timothy - welcomed the opportunity to work with Hawkins again, who he previously appeared with in the critically-acclaimed Royal Court play Constellations in 2012. Director Matthews saw the pair in the production, and "thought it would be nice to put us in this film".

"One of the best things about doing this was being able to reunite with my friend again, I love her," says Spall of his co-star.

He says he didn't have any teachers like Mr Humphreys during his own school days, growing up in South London.

"It's a real shame. I've been thinking about it a lot recently - because my kids are going to go to school soon so we're trying to work that out - and realise how important that is, but I never had an inspirational teacher.

"But I've done all right!" he adds with a smile.

One of Spall's two sisters is actually a teacher, in the reception class of an inner city London school.

"I've got such respect for teachers," he says. "It's an extraordinary profession, and much more worthy than my own."

Since starting out in theatre as a young actor, Spall has managed to combine big and small screen work, with acclaimed performances in TV shows like Black Mirror and Pete Versus Life, and high-profile movie gigs in One Day (2011), Life Of Pi (2012) and Get Santa (2014).

He's recently spent three months in Romania shooting US TV mini-series Sons Of Liberty, in which he plays one of the founding fathers of America, John Hancock.

It meant being away from former Hollyoaks star Du Toit, who he wed in 2010, and their young son and daughter.

"It's tough being away from them," he admits. "Skype is a wonderful thing, but it does make it harder in a way."

He's more than happy to effuse about his own father, who picked up the Best Actor gong at last year's Cannes Film Festival, for the biopic Mr. Turner.

"I've always been so proud to speak about him, especially this amazing time he's having at the moment," he says.

"People do get quite touchy [talking about famous parents]. You've got to make a choice.

"That just comes from a place of insecurity. It's a fear that they're not at the level of the people they're getting compared to."

X+Y is in cinemas on Friday, March 13