The Doctor had set a few things up here and there in the TARDIS ready for his guest. Since he was the one to set up the meeting, he had the advantage and therefore be even a little prepared.
Upon hearing the knock, he smirked to himself and casually strolled over, unlocking the door and opening it to see who was outside. "Ah, Charlie!" he said with a grin. "Or was it Charles? I can't remember if we had settled on what I was to call you" he said, squinting with one eye as he looked at the other man.
How oddly uncomfortable. The way time seemed to flow around this man. Sort of the way it flowed around most people who arrived at Red Oaks. Unnaturally, oddly. It became broken and jagged, fluctuating at differing paces and it made the Doctor mildly squimish.
"Come in, come in" he grinned, stepping back away from the door to let Charles inside. The Doctor loved this part. Where people saw the inside of the TARDIS for the first time.