I'm not quite sure what cues I use to pull off this feat. I'm quite good with voices (even just the general timbre of someone's voice), and the actors' mannerisms and way of moving are often a give-away. Unfortunately, I'm quite bad at remembering names, which means I have a lot of slightly Victoria Wood-esque conversations, along the lines of "Oh look, it's thingy. You know, he was in that film with the guy in the vest. And he was in that other thing with wotshisname. Oh come on, you must know who I mean!" There must be a market for this kind of thing, surely?
1 "He said 'red shirt'! Unnh hnn urr."