Creator Michael Hirst writes all the scripts for History’s Vikings, but actors Travis Fimmel and Alex Høgh Andersen improvised a key exchange between their characters in the recent fall premiere.

The relationship between the two characters—Ragnar Lothbrok (Fimmel) and his son, Ivar the Boneless (Andersen)—becomes central to the “Vikings” story this season.

In a key scene that sparks their journey, Ivar sits next to his father in the throne room, explaining the love and hate he feels for his dad after the older man “abandoned” all his family and the people of Kattegat.

“Perhaps I will explain my actions when we get to England,” Ragnar says, slyly inviting Ivar, who since childhood has not been able to walk, to accompany him on a raid of England.

It seems like an unusual move, considering all of Ivar’s brothers are able to walk—and fight—unencumbered. In the exclusive clip from the scene at the top of this story, Ivar boldly points out the problem in his father’s thinking.

Andersen told me Fimmel went off script for the tense—and funny—bickering between father and son.

“Travis wanted to play around with the scene so I said, ‘Not unless you ask me like you asked my brothers,’ and then I cut him off,” Andersen said. “I think it is hilarious; of course Ivar would do that!

“And apparently it worked out very well. It’s a major scene for their relationship, really. I loved shooting that scene.”

Bringing up his brothers in the conversation was a continuation of the battle Ivar has been fighting all his life, Andersen said. Not being able to walk, he constantly has tried to prove he is a worthy warrior, man and Viking—just like his brothers Ubbe (Jordan Patrick Smith), Hviserk (Marco Ilsø), Sigurd (David Lindstrom) and half brother Bjorn (Alexander Ludwig).

Another scene in the fall premiere makes Ivar realize he won’t be able to do the same things his brothers can, and it is a difficult discovery, Andersen said.

“Not being able to have sex with Margarette is a huge turning point for him,” the 22-year-old Danish actor said. “That kind of tells him he will never, ever be a part of them. He will always be different, and it hits him so hard.”

Ivar might be different, but as Ragnar tells his son, that doesn’t make him less. Ragnar recognizes himself in Ivar—especially when his son challenges him in the throne room.

“Ragnar respects him so much,” Andersen said. “What Ivar does in that scene is the exact opposite of all the other brothers. He plays around with Ragnar. It’s usually Ragnar who does that to everybody else, right?”

Also like his father, Ivar “understands people; how they act and how they think, and what they do,” Andersen added. “And he’s very good at manipulating.”

Ragnar’s belief in Ivar seemed to start as soon as he returned to Kattegat. He feels out all his sons, but according to Andersen, he recognizes how strong Ivar has become because he has been fighting to thrive in a very physical Viking culture despite his physical challenges.

While Ragnar saw weakness in the other brothers’ characters, he saw internal strength, curiosity and fortitude in Ivar.

“I think that is more important than being able to fight with a sword,” Andersen said of Ivar’s strong points. “You will see Ivar becoming a very, very, very smart and intelligent military strategist.”

Alex Høgh Andersen says Ivar learns a lot from his father when they travel to England this season. New episodes of “Vikings” air at 9/8c Wednesdays on History.

More Vikings:

The Vision preview

Vikings fall season premiere recap