Moffat thinks it’ll be fun seeing Clara cope with the Doctor being completely different…
…”I think the fun story will be – and we have the opportunity here – is this is what regeneration can do to you. He can be very, very different.”
Didn't Rose and a few other companions go through that already?
No, not really. Like I said, the differences among the past three Doctors have been minor compared to the differences between consecutive Doctors in the original series. They've all been relatively young, good-looking, and manic with a mix of brooding and comic attributes; the differences have been more or less shifts in emphasis. It's nothing like the shock Ben and Polly went through when the Doctor they knew as a crochety, proud old gentleman turned into a feckless, Chaplinesque eccentric, or what Sarah Jane and the Brigadier had to cope with when the elegant, mature, authoritative Doctor they knew turned into a madcap iconoclast spouting nonsense, or what Peri faced when the gentle, sensitive young Doctor she knew turned into a bombastic, aggressive, unstable egomaniac who tried to strangle her within minutes of his regeneration.
Along those lines, I've occasionally wondered quite how, and when, Benton found out/was told/realised that the tall, arrogant chap working as the Brig's scientific adviser was the same person as the short, funny chap he'd met in passing during the International Electromatix business, not just someone else with the same call sign/codename.
Must have been a bit of an eye-opener, possibly explaining why nothing ever shocks him again, and why he's far more of UNIT's inner circle than his rank would dictate.