"I can't say enough about how great and diverse a group of women we put together for him. They are very strong, smart, and independent, business owners and lawyers and military," Harrison told Yahoo! TV.
Harrison said it helped that Viall, a 36-year-old former software salesman, wasn't picky about a woman's looks going into the season.
"A lot of the Bachelors and Bachelorettes have a type. We have to break them down and get rid of that type in their head because that obviously hasn't been working for 30 years. They need to change course," Harrison explained.
"The good thing with Nick was that he was pretty open-minded. He didn't say, 'I want blondes or brunettes only. '"
Viall apparently offered up a pretty vague and general description when explaining to producers what he was looking for in a future wife during the casting process.
"One thing he did say was that he is attracted to confidence. He likes a strong-willed, independent woman who knows what she wants in life and love," Harrison revealed.
The cast includes Rachel Lindsay, an attorney; Danielle "Lo" Lombard, an owner of three small businesses; Danielle Maltby, a neonatal nurse; and Taylor Nolan, a mental health counselor. Viall also just so happened to give his First Impression Rose to Lindsay, who is arguably the most professionally accomplished woman in the cast.
"This is not the recipe for a simple, easy house," Harrison teased.
"When you have a bunch of strong, opinionated women who are used to getting their way and being successful and suddenly asking them to get in line and wait for their turn, it does not go over very well. Typically things start off nice and congenial, but then they decide they want their time and stop being polite."
Although Harrison told Yahoo! TV it's going to be a "roller-coaster season," the host insisted Viall is "sincerely" hoping to find "someone to spend life with."
"[Nick] has been incredibly unlucky in love and has had his heart broken on national TV so many times," Harrison said, referring to Andi Dorfman and Kaitlyn Bristowe's seasons of The Bachelorette when Viall finished as a runner-up both times.
Viall developed a bit of a bad-boy reputation from his stints on The Bachelorette, mainly due to his aggressive dating approach and promiscuous nature, but Harrison explained he was not exactly "an unconventional choice" for the leading role.
"I would argue that in a lot of ways he is actually a conventional pick for the show," the host said. "We typically look for someone who has been on the show, who we know very well, who is interesting to fans, and who has been unlucky in love but is still sincerely looking for it."
The Bachelor's 21st season premiered January 2 on ABC and concluded with Viall handing out roses to 22 out of the 30 bachelorettes.