Maybe have the characters interacting somehow. Like simply looking at each other or something might make come to life a little bit. Maybe the black and white character and the bald character is looking at the blue character because the blue character looks sad. Not to sure but looks great!

I agree with what Andaries said, plus I would also include a neck of some sort. I know these are Doodles, but a simple neck would really solidify the sketch in my opinion. This kind of looks like a watercolor affect, and with that, the edges around the chin and very messy, and should be cleaned up a bit. Other then that, I really like what you have done here. Keep up the good work.

I think they really turned out great, there is some room for improvement though. You are using a relativlely stylized form of drawing, what would help in giving a bit more character is changing up the form a bit. The chins of all of these persons are looking very similar, maybe try some bulkier, thinner, rounder, sharper etc... Apart from that I noticed that the placement of the facial features is a bit off sometimes, I know that these are caricatures, but giving them some level of form will really help making them look more believable. But I am a real fan of the shading, especially on the middle one and the facial expressions all look very sincere and different, a lot of people like myself have the "dead expression" problem, so kudos to you! ^^

Depends on your goals... where do you wanna go with your art?

I think 1st one is reaaly good