As previously mentioned, Seo first came into the limelight when he joined Superstar K in 2009. But music has always been something that the actor is passionate about. In fact, he has loved music since he was 10 and was that kid who really wanted to perform in front of an audience. He lists Kim Jung-min, a Korean rock musician and actor, as his role model growing up and decided that he wanted to become a singer.

Seo loved music so much that he eventually took up applied music at Daebul University while auditioning for entertainment agencies including JYP Entertainment.

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In 2012, Seo tells Korea JoongAng Daily in an article by Carla Sunwoo that he struggled with bulimia, an eating disorder, which started when the judges at an audition for JYP Entertainment warned him about his weight. "At the initial audition, they said that I should lose some weight...From that point onward, I became obsessed with weight loss. After every meal, I’d vomit so that I could lose weight," he says adding that when he returned to his agency after losing enough weight, he got rejected again because his "singing had worsened".

He presumes it was due to the damage to his vocal cords due to vomiting. Years later after passing these roadblocks, he won the grand prize at Superstar K, which helped him regain his self-esteem and confidence.

Aside from his weight, he also thinks he is an ugly duckling at times, saying "I personally think I have a weird face. Sometimes I look handsome, and sometimes I look ugly. I think the director [of Pipeline] thought highly of this strange visual of mine. He also said that my eyes and aura attract people. I was really thankful for his compliment" at the Pipeline press conference.

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Contrary to his role as the last surviving heir of a chaebol chairwoman in the 2016 series Shopping King Louie, Seo's life is more of the rags-to-riches type. His father was reportedly a welder and his mother used to collect boxes and recyclables from the garbage in order to earn money so she can feed him and his younger sister. He said he didn't even have KRW10,000 (HK$70) in his wallet while in college.

To this day, he looks up to his mother as his role model, saying "My mum went through difficulties ever since I was young... Of course, my father is my role model as well but the reason I picked my mother is that she went through many difficulties picking up recycling papers to raise me...Even after I debuted she continued that work. I told her to stop and rest comfortably but she said she didn't want to because if she rests and stops working, her body would grow sick and she told me that enjoys meeting new people".

It was reported that he gifted his mother with the now-closed Coffee Natteurae, a coffee shop located in their hometown. He also opened Sojindam Ulsan, a restaurant managed by his sister. "I think I resemble my mother in that aspect. I can't take a break or rest. When I do, I just want to work again. So she's my role model", he adds.

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This entertainment multi-hyphenate has bagged one too many awards for his outstanding performances on television. In 2012, he got the Best New Actor award at the 5th Korea Drama Awards for his role in Love Rain as well as the Best Couple Award (together with Apink's Jung Eun-ji) for Reply 1997. Seo In-guk and Jung Eun-ji's song, All for You also won three-peat Best OST awards at 14th Mnet Asian Music Awards, 4th Melon Music Awards and 1st KDRama Star Awards in the same year.

His acting talents were also recognised at the 2013 SBS Drama Awards (New Star Award, Master's Sun), 2014 KBS Drama Awards (Best New Actor, The King's Face) and 2016 MBC Drama Awards (Excellence Award, actor in a miniseries, Shopping King Louie).

While his acting skills aren't questionable, Seo's piercing eyes also convey a lot of emotions making fans root for him to (finally) get the Best Actor award at the 58th Baeksang Arts Awards next year.

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Like the rest of the main cast of Reply 1997 who used authentic Gyeongsang dialect for their roles given the Busan setting, Seo didn't have a problem doing the same as he hails from Ulsan. Undoubtedly, Reply 1997 launched Seo's career as he tapped for the lead role when he was just a rookie actor. It cemented his potential as a leading man and earned him a nomination for Best New Actor at the 49th Baeksang Arts Awards.

After the conclusion of Reply 1997, Seo admitted that his "accent [came] out" when he started filming his new drama, Sons where his character was a student from Busan.

"There was a time when the sound director [for Sons] asked me if my character Yoo Seung-gi was from the countryside and gave me a warning," he said during the press conference. "I think when I get excited now, my accent comes out so I have to be careful", he added. 

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Other than the upcoming movie, Pipeline, Seo has only acted in one movie which is 2013's No Breathing, opposite Lee Jong-suk and Kwon Yu-ri. However, he did have a cameo appearance in the Chinese romantic-comedy, I Love That Crazy Little Thing in 2016.

In Pipeline, Seo stars opposite his Doom At Your Service co-star, Lee Soo-hyuk where he takes on the role of Pin Dol-yi, a drilling engineering. He will be leading a heist to steal oil hidden in a tunnel hundreds of meters underground.

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Seo did enlist in the military in 2017 and even composed the song, Better Together and released a music video to bid his fans farewell. Four days into his training, he suffered from a fractured ankle and was reexamined to see if he was fit to continue. He was eventually diagnosed with a bone and cartilage disorder and was subsequently exemped from the training.

Shortly after, Seo wrote a letter to his fans to apologise and to address the speculation that he was trying to evade military service.

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