Depends on how secondary they are, I suppose. I have a small supporting cast of continuing characters in my "Mitch King"private detective novel series, and those I flesh out with some detail. I give them a "bio" and spend time coloring them in, as I go along (not all at the same time however, which would be boring). These are:
Detective David Meierhoff, my protag's best pal. David's smart, educated, an interesting guy.
Det. Lt. Joe Duggan, the mentor and fierce top cop.
Antonio Villarreal (Tony Vee) a big ex-Oiler lineman who's an enforcer.
Other continuing characters are more tertiary, such as Mitch's attorney Donna Boudreaux, his college buddy Gregor Yevshenko, and others.
The "real" secondary characters are less meticulously drawn. These are the people involved in the case, like informants and victims and supporting-cast cops.
But in all cases I try to make them believable, since I'm writing a very realistic series of novels.