When I try to write about new species, and they're part of the ordinary world of the novel - but obviously completely unknown to the reader, then I actually have to know all of their peculiarities much better than I would have had to know if they had been new, unknown species to the world of the novel.
Some of the things about people we all know well on a subconscious level, so we don't have to worry about having to describe certain objects, motivations or even physical reactions.
Same thing should apply with new species you create.
The problem is that with the new species only you know about, it's easy to forego millions of years of "their" evolution (and potentially even miss the lessons from the evolution of humans as something the reader will likely reference in their mind) to arrive at a completely implausible and flat concept.
Hence, I strongly believe it still pays to do this kind of work, but not for the story itself, but as part of research for it.
Write it well and thoroughly, maybe take whatever is already part of the story and connects to the species, and write it down elsewhere in your "research" materials, expanding more on it.
Ideally, if you are serious about it, you will build a guidebook on the species, from history, to physical features, emotional composition, etc. etc.
Of course, you could always totally wing it too!