Most writers spend a great deal of time fleshing out the hero and heroine giving them flaws and virtues, conflicting likes and dislikes, turning them into real and complex people. But sometimes there is a tendency to spend less time on secondary characters making them one dimensional and maybe even stereotypical.
James Scott Bell in his article "The 5 Cs of Writing the Great Thriller Novel" written for The Writer's Digest advises: "Look for ways friends can become enemies or betrayers." This can create undercurrents and secondary themes. Bell suggests a character grid.
Susan Marcus Robert Carmel
Then fill in the blanks.
Susan Is jealous of the attention Marcus gives Carmel.
(Susan is secretly attracted to Marcus.)
Robert Knows Marcus has an illegitimate child
Many times creating characters who are opposites gives many opportunities for both humor and conflict. Think of Felix and Oscar on The Odd Couple or The female Salvation Army worker and the gambler in Guys and Dolls.