I've started, so I'll finish: why is your 'Mastermind' subject so daft?

John Humphrys, famous for grilling Cabinet ministers, is to interrogate contestants on their reasons for choosing obscure, often absurd, specialist subjects when he takes over as quizmaster of Mastermind, which is relaunched tomorrow night.

Not content with the rigours of the specialist and general-knowledge rounds, Humphrys will use the interval between them to browbeat the contestants about why they opted for their chosen subjects in the first place.

In one programme, he will be seen mocking someone for choosing to answer questions on Sixties and Seventies motor cars. In another, he will quiz a contestant about how he came to be an expert in the "miserable" music of The Smiths. Other subjects include the history of The Simpsons cartoon series.

Phil Parsons, Mastermind's executive producer, says the extra, unofficial "question round" is intended to help imprint the personality of its new presenter, who is best known for his grilling of MPs on Radio 4's Today programme.

"We want to get as much of John's own character across as possible, so ... he will do mini-interviews with all the contestants," he said. "He does one really great interview with someone who chose to answer questions on cars of the Sixties and Seventies, a period when there were a lot of crap cars around, and he asks someone else 'why did you choose the music of The Smiths? It's just miserable'."

Aside from retiring Magnus Magnusson, who presented the series for 25 years until it was axed in 1997, the BBC has also replaced the iconic black leather chair contestants used to sit in. In its place will be a chic retro model from the "soft pad" range originally launched by Charles and Ray Eames in 1969 - a style favoured by, among others, Peter Mandelson and seen on the TV sitcom Frasier.

But the sombre tone of the programme's theme tune will remain intact, as will Magnusson's famous catchphrase "I've started, so I'll finish".