In terms of building a character, what should guide you is what the story requires. If you need a mad scientist to make Frankenstein's monster, but the person you meet is nice and well-spoken, do you then change your whole story?
It is not a bad idea to notice things about people, and incorporate them into the story when you need them. Habits, pose, movement, dress, verbal tics. For example, J.K. Rowling had an unpleasant teacher who had a preference for twee things. The unpleasantness bound with the style became a defining trait of Professor Umbridge. But the impetus to incorporate the particular person into the story came from the story, not the other way round. It's not until the 5th book that Umbridge comes in.
You can go to a professional for professional knowledge, that makes sense, but as far as being a person goes, we're all professionals in that. Why should you base your character on a guy you haven't even met yet?
There is another element in play: if you tell a person you're going to base a character on them, first they might not want this. They might well be concerned about how you represent them, want to maintain their privacy, or prefer to avoid this for some other reason. Second, if they don't mind, they might well be self-conscious during your interview, acting unnaturally. How does that help you write a character? Third, suppose you promise a person to base a character on them, but then for various plot reasons, you decide you need the character to be different. How are you going to explain that to the real person you've made a promise to? Fourth, suppose you base a character on the person, but then you need him to do something immoral - commit a crime, lie, create a monster, whatever. How do you think the real person is going to feel?
Note that most fiction books contain a disclaimer on the copyright page, along the lines of
All the characters in this book are fictitious, and any resemblance to actual persons, living or dead, is purely coincidental.
This is done exactly to protect you from all the trouble I've listed above (and more similar trouble). Which proves that there is trouble to be had with basing a character on a real person without their consent. (And of course, you cannot very well put such a disclaimer in your book if it's a lie.)
And yet another thing, though maybe this should go without saying. Do your homework before you meet a professional in person. Find out as much as you can about the subject you're going to be asking about, and prepare the specific questions you want to ask. The person's time is valuable. Don't waste it. If the hour you have with a scientist is spent on him explaining you the basics, you've gained nothing you couldn't have found yourself. If you already know the basics and can understand what he's talking about, you can use the time to gain an advanced insight that can make your story better.