The Terileptils are a mild step forward in terms of how alien races are realized in Doctor Who because of the preliminary use of animatronics. As far as I can tell, these technical advances amounted to just about an occasional lip curl and brow furrow, but it's a start.
The actual jaw movement of the beast, though, appears to be performed by the actor inside the costume, Michael Melia. Unfortunately, while Melia does a fine job with providing the voice of the Terileptil leader, you can hear his voice get muffled by the headpiece of the costume. If some effects could have been used to treat Melia's voice, the problem wouldn't have been as noticeable. Without them, though, it just sounds like a man in a costume, defeating all which the new animatronics rig was striving to achieve.
The most notable incident that occurs in this episode, though, is, of course, the destruction of The Doctor's sonic screwdriver. The last holdover of the invulnerable, pre-JNT days (omnipotent robot dogs and intelligent companions having preceded it), the classic series, surprisingly, stuck to its guns until the bitter end by taking away The Doctor's most trustworthy tool. It's fitting, though, that after the Terileptil leader destroys the screwdriver, The Doctor is trapped in his cell for the remainder of this episode and well into Episode Four.
John Nathan-Turner saw the sonic screwdriver as an easy out for The Doctor, a catch-all solution to get out of any problem that writers for the series tended to abuse. But The Doctor rarely, if ever, "saved the day" using the sonic screwdriver. The only thing it consistently did was open doors, thus extricating The Doctor from something as mundane as a prison cell or locked room, thus allowing him to rejoin the action. Without the screwdriver, The Doctor wasn't only kept out of the action, he was kept out of danger. The destruction of the sonic screwdriver began an irregular pattern of keeping the hero of the show from being able to fully participate in the action, a move that could be quite injurious to the format of the programme.