Polly, do you mean a blanket baby is easygoing? My first was completely easygoing as a baby (and I am told I was too). She probably would have fit your definition, though I'm not sure about looking sleepy, but in general she was content to observe and be still often. Didn't attempt to crawl until 10 months but nonetheless walked by 13ish months.

She still is a really easy kid at 2.5, because she's incredibly verbal, and has been since starting to talk. I can just explain to her what we're doing or when something is going on to scare or upset her, comfort with her with very few exceptions. Like someone mentioned, I pretty much talk to her in simplified adult terms and when I have tried doing this with others her age or even older, I get blank looks back at me.

My younger DD (now 10m) has always been called alert, and is much more intense in general, particularly when upset. I was frequently told she was very alert as a baby etc, and she required constant entertainment from the get go. We have had predictive type comments from people who know her about how smart they think she is or will be. Time will tell but I'm sort of afraid, to be honest, to find out if they're very right (or likewise very wrong).

I'm not sure there is a rule in this.