There’s something magnetic and irresistible about the vibrant woman with the wild red hair. She makes the Doctor more than a little uncomfortable.
She acts like she knows him, even though he’s sure he’s never met her in any of his lives. She calls him Sweetie and when he tries to find out what they are to each other, at some indeterminate time in the future, she won’t tell him. All she’ll say, with a teasingly flirtatious expression on her face, is “Spoilers!”
Nevertheless, he’s sure of two things about Professor River Song; she’s trouble, and he’s flirting with disaster.