Oh yes, indeedy! Several times in fact. The first romantic novel I attempted, a contemporary western, I cannot exactly pinpoint where the heroine's story morphed into her grandmother's. From researching ranching and rodeo events, and in the processing meeting cowboys, stock contractors and rodeo riders who were endlessly helpful, I found myself researching pre-war Montreal and then the French Underground during WWII. I finished writing that book but because there were so many huge leaps from one place and time to another it was very disjointed. At that time I was too much of a newbie to know what to do with it and didn't want to do what several professionals advised which was to cut 25,000 words and relocate the story from Southern Alberta (where I live) to Montana or Wyoming which at the time I had never visited and knew little about. That story still languishes (figuratively speaking) under the bed.
When I started writing my first Regency romance, His Dark Enchantress, my hero's sister kept intruding. She was so pushy I kept asking myself who's story was I telling? Juliana (the sister) came front and center ahead of Emmaline (my heroine) on every page. I finally gave in and promised Juliana her own story, which I am now in the process of writing. I don't think of myself as a pushy person but in some corner of my psyche I must be as Juliana is, in fox hunting terms, a 'thruster' or a person who rides at the front of the field and too close to the hunt staff or hounds.
One of my favorite Regency novels is Frederica by Georgette Heyer. The copy I have is much-loved, beginning to be a bit tattered, first edition. I still read it from time to time and find it as fresh and funny as the first time I read it. A secondary character in that book, which I am as comfortable with as an old friend, is the Marquis of Alverstoke's secretary Mr. Charles Trevor. Charles is an absolute paragon of efficiency without being the least bit stuffy and, in fact, Frederica describes him as 'an excellent young man'.
Secondary characters can bolster the hero or heroine, they can be a good friend of either. They can be smart and slightly caustic, or a bit of a buffoon as a foil. They all have their place and in my opinion most books are better for including them.
I hope you'll visit these fine authors and take a look at how they feel about secondary characters:
Fiona McGier http://www.fionamcgier.com/
Anne Stenhouse http://annestenhousenovelist.wordpress.com/
Skye Taylor http://www.skye-writer.com/blogging_by_the_sea
Beverley Bateman http://beverleybateman.blogspot.ca/
Judith Copek http://lynx-sis.blogspot.com/
Connie Vines http://connievines.blogspot.com/
Helena Fairfax http://helenafairfax.com/
Marci Baun http://www.marcibaun.com/
Rachael Kosinski http://rachaelkosinski.weebly.com/
Hollie Glover http://www.hollieglover.co.uk
Dr. Bob Rich http://wp.me/p3Xihq-CZ
Rhobin Courtright http://www.rhobinleecourtright.com/