Writers tend to lean heavily on certain words or expressions. Nothing is wrong with this at the early stages because it helps get the draft down.
But especially if the same words appear again and again in most of the characters' speech patterns, some tweaking is in order to distinguish the voices and add more variety to the prose.
A few thoughtful minutes, an awareness of the tendency, and the search function are usually all that's needed in remedy.
I myself am particularly guilty when it comes to "oh" and "sort of."
"E-Mail Submissions Made Easy" by Brandy S. Brow from the Institute of Children's Literature.
Jan Thornhill: official author/illustrator site features biography, information on books, illustration techniques with step-by-steps, school visits, links, etc. Thornhill's books include The Wildlife ABC & 124; Before & After: A Book of Nature Timescapes; The Rumor; and Over In The Meadow.
Madeleine Thien, author of The Chinese Violin (Whitecap Books, 2001), talks about writing, the beauty of reading, and the influence of her parents and of Malaysian culture in her work in an interview by Laura Atkins from Papertigers.org.