I've been asked a number of times about the differences between the two lines I've written for: Harlequin Intrigue and Harlequin Romantic Suspense.
As their senior editors, Denise Zaza (Intrigue) and Patience Bloom (HRS) tell prospective writers in their linked Q&A chat, the two lines are definitely not interchangeable.
In a nutshell, I'll summarize what they're saying, adding in some examples from my own experiences:
1. Intrigue is a bit shorter (65-70K), more focused on the criminal investigation/procedural aspect of crime solving, and extremely "hero-centric." The more alpha the male is, the better. The romance element is less predominant than the intrigue focus, sensuality may vary but must be fully integrated with the crime plot, and love scenes receive less emphasis, if they're in the book at all. (My most recent Intrigue, Relentless Protector, involves a very suspenseful hunt for the heroine's missing five-year-old, and I can tell you, s…