An interesting John McWhorter post at Language Log:
Tonight an actor said AND THAT’S WHY I’LL TELL THEM AS SOON AS I CAN in rapid, casual style, but he inserted a note of falseness by pronouncing THEM as “THEHM” rather than the way any native English speaker would pronounce it in that sentence, “THUM.” “THEHM” did not aid clarity in any way — if he had said “THUM” the audience would have still known exactly what he was talking about. He said “THEHM” out of a sense that this is what the word “really is.”
But actually, “THEHM” is just the full form. “THEHM we can talk about,” for example. “Me and THEHM went yesterday.” But just as often, English makes use of a second form, the short one, THUM. By no means a lapse or mere static, THUM is absolutely required of anyone who wants to speak English without sounding like a Martian, or a competent but not quite acclimated newcomer to the language. But because our writing conventions “unravel” the language and transcribe both the full and short forms as THEM, the actor is often distracted into supposing that always saying “THEHM” is good form, “rendering the text properly” Actors erupt in these phony “THEHM”s all the time — I have even heard actors pull this when spouting the vibrantly choppy, earthy vernacular of David Mamet plays.