Well, on stage and in film, you have to have your mind sharp. Unlike most foreign characters played by actors, especially Americans, who couldn't do it to save their own skins, The Doctor has a specific accent and David is Scots like myself. When your off your face as I think you're suggesting, you would find it a little extra hard to maintain character. I know this because I was in the game many years ago and almost every character I played was from somewhere else. In an ongoing TV show it has to be maintained day after day and week after week. A real accomplishment.
His date is to the theatre. Not someplace where you'd have a quick joint or snort before going to. The pictures are probably done on a digital camera that can be unflattering at the best of times and it is at night so his eyes would be very dilated.
Actors in general have to be very clean with their bodies and keep their bodies and minds sharp. If they don't, they stop getting work.
I think it is important to be careful of what you say of someone as it may reflect badly on oneself.