Are you my Son?

Leontes (I.ii.183-195)

Too hot, too hot!
To mingle friendship far is mingling bloods.
I have tremor cordis on me: my heart dances;
But not for joy; not joy. This entertainment
May a free face put on, derive a liberty
From heartiness, from bounty, fertile bosom,
And well become the agent; 't may, I grant;
But to be paddling palms and pinching fingers,
As now they are, and making practised smiles,
As in a looking-glass, and then to sigh, as 'twere
The mort o' the deer; O, that is entertainment
My bosom likes not, nor my brows! Mamillius,
Art thou my boy? 

In The Winter's Tale, we see Leontes struggling with his suspicions that his wife, Hermione, is having an affair with his childhood friend Polixenes. Like in Hamlet, Shakespeare has a tendency to have his characters act. Leontes puts a "free face ... on" so that he may observe his wife and Polixenes without them knowing. He knows that if they see his suspicions, they will shift their attitudes so that they are inconspicuous. Even now, they seem to be careful, "making practised smiles" so that they, in Leontes's mind, don't give anything away. Leontes himself is seen absorbed by this idea, much like Othello was towards Desdemona and Cassio. He claims that "To mingle friendship far is mingling bloods." Since Hermione is pregnant, it's almost a direct statement of his fears. Because Polixenes has been their for so long, he and Hermione had (presumably) had opportunities to be close and develop friendship. In Shakespeare's time, a mention of mingling bloods was an almost outright reference to sex. So if Leontes's thoughts direct their friendship to the  combination of their blood, he certainly suspects that Hermione's unborn child is Polixenes's. He even goes as far as to question Mamillius's paternity by asking "Art thou my boy?"

It will be interesting to see how this plays out with so much DRAMADRAMADRAMA already!