In vino veritas

Liam paused, holding the bottle of wine at arm's length. Behind him, the soft hubbub of conversation filled the restaurant, and in front of him, across the table, Miriam watched with bright eyes and trembling lips. Liam jerked his arm slightly upwards, a clumsy toast to Miriam, and then pulled it in and tipped the bottle to drink straight from it. He swallowed twice, then placed the bottle back down on the table. A thin dribble of red ran from the corner of his mouth and stained the collar of his white shirt.
"Well?" Miriam's voice trembled like her lips. In her lap, concealed by the snowy-white tablecloth, she folded her hands over one another repeatedly.
"There's the oddest flavour to it," said Liam, his voice slurring a little. "I'm more than a little tipsy, so it's hard to be precise, but it tastes kind of... bitter."
"I've heard that said." Miriam relaxed a little, her face smoothing out and her hands falling still for the first time all evening. "I've never wanted to taste it myself, but I've heard it said that it can be very bitter."
She paused, looking down at the table, at the space where plates had yet to be placed. "Or very sweet."
"I still don't understand, Miriam. Why did you want me to taste this wine?"
"It's a special wine." She looked up and smiled, little crow's feet forming at the corners of her eyes.
"You remind me of my mother when you do that," said Liam. "She had a way of being coy with people. It got her killed."
"You've said that before. I did want to ask you about that. How did your mother die, Liam?"
"She ran out of the house, running away from William, and out into the street. A cyclist swerved to avoid her, and she flinched away and her foot caught in a pot-hole. William came running out of the house and threw the knife at her, and she dragged herself out of the path of the knife and under an oncoming bus. The bus driver was looking the other way."
"William was your older brother?"
"William was my older brother's imaginary friend."
Miriam looked back down at the tablecloth, and her hands started rubbing one another again. The secret ingredient to the wine was a truth-serum; anyone drinking it would find it impossible to not tell the truth for anywhere up to five hours. And now it seemed that Liam's little madness, his devout belief in his elder brother's imaginary friend was somehow real.

Or Liam was actually incurably mad.

Miriam signalled to the waiter for the bill.