[Clint Lando]

Is Kevin Costner's character based on a real person?
Not exactly. In researching the Hidden Figures true story, we learned that Kevin Costner's character, Al Harrison, is based on three different directors at NASA Langley during Katherine Johnson's time at the research facility.

Did Katherine Johnson feel the segregation of the outside world while working at NASA?

No. "I didn't feel the segregation at NASA, because everybody there was doing research," says the real Katherine G. Johnson. "You had a mission and you worked on it, and it was important to you to do your job...and play bridge at lunch. I didn't feel any segregation. I knew it was there, but I didn't feel it." Even though much of the racism coming from Katherine's coworkers in the movie seems to be largely made up (in real life she claimed to be treated as a peer), the movie's depiction of state laws regarding the use of separate bathrooms, buses, etc. was very real. African-American computers had also been put in the segregated west section of the Langley campus and were dubbed the "West Computers." -WHROTV Interview

Did they really think Katherine G. Johnson was the janitor there to empty the garbage can?
No. This does not appear in Margot Lee Shetterly's Hidden Figures book, on which the movie is based, and seems to be an element of fiction created by the filmmakers.

Is Jim Parsons' character, NASA engineer Paul Stafford, based on a real person?
No. In fact-checking the Hidden Figures movie, we learned that white collar statistician Paul Stafford, portrayed by Jim Parsons, is a fictional character

Hidden Figures Movie vs the True Story of Katherine Johnson, NASA