He avoids making eye contact when he's bluffing. Zeref's voice would be heard in Erza's head. He'd long since mastered telepathy, on the scale of being able to issue commands simultaneously to entire regiments of the Imperial military all at once. It would feel completely different from when Warren does it. Beyond likely having learned the magic in completely different ways, the two weren't anywhere close to being on the same league as mages; with Zeref, there was no subtlety, no gentleness, no warning, just the voice of someone who was used to micromanaging hundreds of thousands of people all at once like a skilled real-time strategy game player, focusing on only one person in the room... well, two, if you count Mavis.

He thinks his mask hides it. It doesn't. I can see his shoulders tense up every time he looks at his cards. He then looks at his drink. He's stressed, and he thinks the alcohol would help him. Yes. Zeref is helping, though not necessarily helping her cheat. He can't see their cards, after all. If she were to scan her surroundings, she'd see Zeref sitting next to Mavis at a different table. They've been the last two people at one particular poker table for almost an hour. Everyone else had been beaten, but as usual, the two can't seem to win against each other.