Theo simply nodded at Severus' words. He'd always been a man of study, a reason why his library was now twice as big as it once had been. Books were after all far more reliable than men most of the time, but in this quest, they had over the years proven to not be enough. Not that he didn't still read them vigorously.
For a moment when Severus told him to begin he remembered his first times, the way he'd had to prepare himself and the lack of strength and finesse his attempts had been. He was better now than he had been all those months ago, much better, and could easily focus on the blocks and barriers Severus put up as he entered his mind, looking for the weak points while being prepared for Severus to either throw him out or trying to enter his mind instead.