He’s sizing up a worthy adversary with respect and amusement for the coming chicanery as he puffs his pipe and ponders the potential gains to be won.
That was fun!
I see a look of determination. He’s facing some sort of challenge, and is determined to do his best to overcome it.
Whenever I see it I think about the title “The Curious Case of the Humanoid Face… on Mars” by Richard Hoagland, from the November 1986 issue of Analog Science Fiction and Fact. (Hoagland thought it was one, but it was the other.)
I just showed the picture to my kid, who said, “Mischievous. Conniving. Like he’s about to rub his hands together and say ‘You Fell Into My Trap’.”
How old is the kid?
Reading the above, I agree.
However, after reading the first post, I was at a loss.
When I go to a museum, such portraits are often what I like best. And I look at the them and decide which I like best. What I don’t do is try detecting the emotion – because I think I am bad at it. How portraits can interest me when I’m bad at picking up the emotions is an open question.
If anyone wants even more of a challenge along these lines, here are six high quality portraits that I saw several times locally, and admired, before they were sold:
# Thomas Eakins at St. Charles Borromeo Seminary
The last paragraph here gives far more insight into one of them than I could ever dream of detecting:
https://artmuseum.princeton.edu/story/thomas-eakins’s-right-reverend-james-f-loughlin
Sixteen.
It’s not a face. That’s just your pareidolia talking.
Lasciviousness.
Thanks. And a good judge of character it seems. Congrats on a job well done.
Or at least well done so far to the 3/4ths point. 
I voted “other” and I also think it is anticipation of pleasure of some kind.
So, OP, you’ve had more than 20 responses so far, and 29 votes, are you ready to spill yet?
Mischievous is what immediately popped into my head so I picked amusement as it seemed closest.
This is a detail from the painting Reply of the Zaporozhian Cossacks, which was linked in another thread just today.
Looking at the Wikipedia article, this person is called “The Smiling Soldier.”
I cannot resolve that expression into a smile. To me he looks deeply suspicious, watching his surroundings very carefully. Distrustful, as @Czarcasm said.
I was just curious what other people would think.
Wow, that is quite the painting. A lot to take in.
And no, not a smile.
I voted amusement. I’ll buy smiling.
I thought “satisfaction,” as well, but more in a kind of “ha ha! now things are going my way.” It could be because of someone else getting their comeuppance, or falling into his trap, or who knows what.
Still reading through the thread:
More or less exactly this, though I read it as pleasure at witnessing someone who has just fallen into said trap.
Amusement and contempt like he’s about to steal someone’s lunch money.
To me there’s a hint of fear / trepidation in there, as in: “I just did a wicked thing and I liked it - wonder if it’s going to bite me in the ass?”
I saw a mix of anger and amusement; and also that he seems to be looking intently at somebody, or maybe at something.
Having read post 33:
that fits. He’d be angry at the sultan, and amused at their reply.
He looks like he’s plotting something and feeling really good about it.