EXCLUSIVE: 'Vikings' Star Alex Hogh Andersen Talks Shocking Season Finale, Learning From 'Genius' Travis Fimme

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WARNING: Spoiler Alert! Do not proceed if you have not watched Wednesday's episode of

After the death of Travis Fimmel's Ragnar Lothbrok halfway through Vikings' fourth season, the series' successor seemed to anyone's game. But after Wednesday night's explosive season finale, the question wasn't who will succeed Ragnar, but rather how far he will go, as Alex Høgh Andersen and his character, Ivar the Boneless, took charge.

ET hopped on the phone with 22-year-old Andersen ahead of the shocking finale, where he opened up about what he learned from Fimmel before the star's series departure, the pressure of joining the History Channel show and his character's "violent" future.

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"You can't really overthink the whole thing, because otherwise you will screw yourself up, because it is that big," Andersen said of joining Vikings with only a few Danish TV credits to his name. "You just have to go with the flow, and rely on the people that are there to help you."

"I am pretty new and green, and I didn't quite know what I was getting into, and it has been such a learning experience and it still is," he added. "Working with Travis, straight from the beginning, that made everything for me."

Andersen only filmed a few episodes with Fimmel before Ragnar's death, but says the short time they spent together was "advice in itself."

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"Working on the scenes on the day with him, some days the day before, just talking to each other over the phone about it was a game changer," recalled Andersen, now filming season five of Vikings. "I really quickly picked up how everything was going down, how you work on the scenes and how you talk about them, what you could get through as an actor, having your own thoughts about the character and the scene and all that, because it is a creative process and you have to compromise with the director and [creator/writer] Michael Hirst."

Fimmel's creative process -- which Hirst previously told ET included inventing scenes, discussing every line of dialogue with him before rehearsals, and even choosing to go a whole episode without saying anything at all -- quickly rubbed off on Andersen, but perhaps not by coincidence.

"He's a plain genius, and he taught me to believe in myself and my own ideas and keeping it logical and real. Especially for us to work together on episode 13, when we are alone in the forest in England, that was just awesome. We both agreed that we wanted to make it relatable and light and create a contract to all the other scenes in the show, just between a father and son, really."

The actors exhibited clear chemistry throughout the show, though it wasn't that connection that scored Andersen the part of Ivar. In fact, Hirst told ET that he cast the young actor, who came to audition for the parts of Ragnar's other three sons, after he exhibited an expression in his eyes that reminded him of Fimmel. "Thanks Michael, no pressure at all," Andersen cracked when he heard of Hirst's comparison.

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"I'll never forget [my audition]," Andersen laughed, describing the 30 minutes he had in the hallway after Hirst requested he audition for the character. "I had no idea who Ivar was, and I had never read any of the scenes that he had… I was sweating. I had never sweat that much in my life."

"It was a scene between Ivar and Ubbe (Jordan Patrick Smith, who coincidentally read with Andersen in the audition), and we rehearsed the scene without the cameras rolling, and I got through the scene. I was like, 'OK, I got this. Just calm down, relax, breathe.' And then of course when they press the little red button the camera, I forgot the first time, I forgot the second time!" he yelled. "But the third time was apparently the lucky one, and I can remember Michael clapping his hands while I'm walking out of the room. It was just a weird day."

Though Andersen remembers clamming up in front of the camera, Hirst said that it was through the camera's lens, observing the gentle pain of a crippled Ivar through his eyes, that showed him Andersen was the one.

"That's interesting," Andersen said, "because it may have helped that I didn’t know anything about Ivar. I was just going logically into the whole thing."

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"I walked into the room and asked, 'What's up with Ivar? Is he in constant pain?' and [Hirst] said, 'Yes, I would think so.' So in my mind, I was like, 'Well, if he has been suffering from this disease for his entire life, being in constant pain, he is used to it, so he wouldn't act it,'" he recalled. "So I put my legs a little to the side, but I was acting in pain through the eye. That's what I did and apparently that's what they went with."

Though Andersen says he's now used to all the crawling around he does on the show as a result of his character being paralyzed, playing Ivar the Boneless didn't excuse him from the cast's intense training.

"They did get the four brothers a personal trainer, and he killed us for like six days a week for three weeks. My body has never been in that much pain before," the Danish actor confessed, adding that he now has an exercise plan to keep up with on his own. "I have to be honest about this, I do skip leg day once in a while."

"It's tough, but every single time he's crawling, it's a signal to the audience that he is dealing with something that they probably won't fully understand, and that he is different, so I prefer to have him crawl once in a while," Andersen said. "I work a lot on keeping his disease in the minds of the audience."

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In the final few episodes of the season, however, Ivar finally gets to join in with his brothers, strategizing and leading the Great Heathen Army from a chariot in a battle sequence that included more extras than Braveheart.

"To be a part of it, personally, it was absolutely outstanding. The amount of extras, the amount of work put into this whole thing, it was a crazy experience. It was a great reminder of what you're doing and how big it is, because you can forget that a little bit when you're just standing in a studio. So it's cool when you get outside with 300 extras and five cranes and six cameras and trucks everywhere," Andersen mused. "It was just f**king mental."

As for the more nuanced aspects of his character -- notably the anger, arrogance, and confrontation with his brothers -- he exhibits in the season's final episodes, Andersen says it comes from struggle.

"Every single thing he does is a compensation for his disease, and him struggling all the time to prove himself," he explained, noting that Ragnar verbalizes his faith in Ivar before he dies. "He is struggling with being crippled and not being the picture of someone who could lead an entire army, the fact that they won't see his brilliant mind. It's the most annoying thing, when you've got the answers, but you're not heard."

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Of course, as viewers witnessed on Wednesday, Ivar's frustrations get the best of him when he dives an axe through Sigurd's (David Lindström) chest, killing him in front of the entire Great Heathen Army.

"It's a major, major scene… he would be the absolute weapon if he could control his anger, and that's something he will struggle with, even in season five. He hates it, because he lost control," Andersen offered. "Despite all the experience he has gained, he has become more of a man, definitely, but still struggles with that little kid inside of him that is just so angry, so hurt."

The audience will also see Ivar attempt revenge on Lagertha (Katheryn Winnick) for killing his mother (Alyssa Sutherland) in season five. "He will never forget that," Andersen stated. "She's the one woman who ever loved him, so he will never, ever be able to settle down with Lagertha alive."

As for how Andersen would describe the Ivar yet to come, he says, "Determined, still angry, violent, but also in love."

"He's completely out of touch with reality and his emotions," he promised.

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