Harrison confessed during an appearance on The Ellen DeGeneres Show that it took some time for Viall to really embrace The Bachelor process after three failed stints on the reality dating show.
"I liked Nick, but now I'm looking at Nick and it seems like he's guarded and he's not sincere. I mean, do you agree that he's coming off that way?" Ellen DeGeneres asked.
"That's definitely not without merit," Harrison noted.
"He definitely did a 180 in the public eye at least, when he was on [Bachelor in Paradise], and I think that's how people got to see the guy we already knew and we loved and that's why we picked him as the Bachelor. But you're right."
But on Paradise, Viall became a sweet friend to all the women and played the hero when trying to protect America's sweetheart, Amanda Stanton, from the polarizing Josh Murray. Viall became a sympathetic and relatable character.
"At the beginning of this show," Harrison continued, "Nick is kind of skating, and he's kind of just going through the paces and going through the motions and doing what he's, you know, probably always done -- his defense mechanisms."
Harrison said Viall is "a good-looking guy" and very "charismatic," so that's probably "been enough for him" in the dating world up until filming The Bachelor.
"And you will see as this show goes on... sometimes you've got to have that come-to-Jesus meeting, [and] I had one of those with Nick. It definitely gets to that breaking point of, 'Look, I need you to show up.' You can't just skate through this," Harrison revealed.
"The Bachelor is a very interesting thing. You can't fake your way through it."
Harrison then compared one's The Bachelor journey to going through rehab, which the host admitted was "a bizarre analogy."
"Nobody comes out of this on the other side unchanged. Every man or woman who's been the Bachelor or the Bachelorette goes through this change in their life," Harrison explained to DeGeneres.
"And you get stripped down really emotionally. I know this sounds silly because it's entertainment and we all laugh and we watch this show, but truly when you go through this, like Nick is doing, he will do what he's always done, but that doesn't work! Obviously it hasn't worked! He's been on the show four times."