An interesting exercise, recreating the "live drama" staging from the original fifties series (with occasional pre-filmed inserts), and basing it off the scripts from that time. The story is largely the same: a British space expedition returns to Earth with only one of the three astronauts sent up still in the capsule, with the survivor, Victor Caroon (Tiernan), unable to provide much information and behaving increasingly oddly. Professor Quatermass (Flemyng) and Dr. Briscoe (Tennant) gradually realize the spaceship encounted something extremely alien when in space, that it's now back on Earth, and has to be stopped before it infects the rest of humanity. Unfortunately, before they work this out, Caroon has broken out of hospital, and is now on a sight-seeing tour around various Central London landmarks.
This was cast before Tennant was given the role of Dr. Who, and in the light of subsequent events, it might have been better to have swapped his role and Fleming: the latter just doesn't have the necessary authority to play Quatermass, though he's trying hard and isn't awful. There's basically no special effects either: while that may have worked in the fifties, with lower audience expectations, it doesn't fulfil the needs of a more-"sophisticated" [quotes used advisedly] modern viewer. In particular, the ending, where Quatermass basically talks to Caroon, seems a severe letdown, adopting the clichéd approach of "love conquers all." While in line with the original, I'd have preferred the more-rousing ending from the Hammer adaptation, The Quatermass Xperiment, involving electrocution.
You can spot a few flubs, such as lines being stumbled, but nothing too obvious, and a lot of the time, you'd be hard-pushed to see anything separating it from a regular drama: it's not like it all takes place on a single set, or adopts other, similarly theatrical conventions. As a production, it's more a novelty than anything, and I'm still unsure as to the point of the exercise. What did the live performance bring to proceedings? I don't have any real answer to the question, and that leaves it simply a fairly well-acted, miscast gimmick.