It's usual for a published book to include a disclaimer, somewhere in the initial pages, to the effect that no character in it is based on any person, living or dead. (Except when it's obvious, like Napoleon, or Queen Victoria). Yet is still fairly common for readers to assume that books rely heavily on portraits of people the author knows, and that if anyone who is acquainted with a writer stands still long enough, they are in danger of being included in a book.
Writing romantic thrillers usually means producing larger-than-life characters, or ones who are undergoing some sort of extreme event or emotional challenge -- not the sort of thing you meet on the high street. My characters turn up, with their own history, inside my head. (And what does that say about the inside of my head?)
My books are basically about emotions ... two people falling in love and fighting danger. Or possibly fighting love and falling in danger. And my characters have to have the special skills and emotional makeup to deal with whatever I've thrown at them. Which does not mean that I'm not a compulsive people watcher and eavesdropper. People-watching, for me, is mostly about body language and movement, that little gesture that signifies love, fear, tiredness, confusion. The way a woman fingers the chain around her neck, or the sequence of moves a man makes when removing a motorcycle helmet. Those are the things that are likely to end up in a book, not a portrait or caricature of a person that I know.
And listening to people talk? I do that for the speech patterns. I've a particular thing about the rhythm of a sentence (too much time in the theatre, listening to Shakespeare?), I tend to read what I've written out aloud to check not so much how it sounds, as how it balances. It's a difficult one to explain, except that sometimes the sentence has too many words and sometimes too few, which has nothing to do with what is being said. Speech patterns are important too. I write a lot of dialogue and it's a good idea that everyone does not sound the same -- so expressions and accent are useful in marking people out. As they grow, characters get a vocabulary of their own, and I discover their favourite words -- if things are going well.
So that's the sort of inspiration I get from people. They never get lifted whole, to inhabit my books. Romantic thrillers are escapism, a variety of fantasy -- I hope my characters act and speak like real people, but they're not based on real people. It's far more fun to make up my own.