How to abort one post-doc sub-project politely Right after starting a post-doc, another post-doc in the new group came and invited me to collaborate in one of his projects (only a side thing; not my main research project; somehow independent from my advisor). I agreed to participate, but recently I've been procrastinating that work because, to be honest, it doesn't seem quite appealing now. 

Of course, I have other projects of my own -which are going ok-, but this one is a bit of a bug in my head. How can I gracefully tell my colleague that I'm quitting?
Bad news is almost always best conveyed:


Immediately
In person


You cannot change that you probably should have said this earlier, but you can prevent it from being further delayed now.

So go find your colleague and tell them you are not going to be able to collaborate with them. Probably best to apologize to them as part of this conversation, too.

A lot of this will depend on how long ago this agreement was and how long you've been procrastinating saying anything. If we're talking a week, it's less important than if this other post-doc has been waiting for you for a month. If it's been a long time, make sure to apologize for taking so long to tell them, too.